feat: async pathfinding with provisional paths and bit-grid snapshots

- Add StandableBitGrid: O(1) bit-packed snapshot (~6KB per 50k tiles vs HashMap overhead)
- Implement two-tier pathfinding: sync for short paths (<64 tiles), provisional+async for long paths
- calculate_provisional_path: capped A* returning path to best heuristic node
- calculate_async_path: A* using bit-grid (Send+Sync, no thread_local)
- prepare_paths system: dispatches provisional paths immediately, spawns async for full paths
- poll_async_paths + splice_completed_async_paths: seamless path transition when async completes
- Entities start walking immediately on provisional path while full path computes in background

Architecture:
  FixedUpdate: prepare_paths → update_wandering_targets → movement
  PostUpdate: poll_async_paths → splice_completed_async_paths

Priority: DF-like pathing (immediate movement) > performance > memory

Cargo.lock

@@ -2335,6 +2335,7 @@ dependencies = [
  "bevy",
  "bevy_platform",
  "bevy_rand",
+ "futures-lite",
  "image",
  "nohash-hasher",
  "noise",

Cargo.toml

@@ -14,6 +14,7 @@ rayon = "1.11.0"
 rustc-hash = "2.1.1"
 ahash = "0.8.12"
 nohash-hasher = "0.2.0"
+futures-lite = "2.6"
 
 # Enable max optimizations for dependencies, but not for our code:
 [profile.dev.package."*"]

src/constants.rs

@@ -4,6 +4,7 @@ pub const TILE_SIZE: f32 = TILE_PIXELS as f32 * PIXEL_RATIO;
 pub const ITILE_SIZE: i32 = TILE_SIZE as i32;
 pub const SEED: u32 = 420;
 
-pub const PATHFINDER_SHORT_PATH_MAX_TILES: i32 = 100;
+pub const PATHFINDER_SHORT_PATH_MAX_TILES: i32 = 64;
 pub const PATHFINDER_MAX_NODES: usize = 5000;
 pub const PATHFINDER_WAYPOINT_THRESHOLD_TILES: i32 = 100;
+pub const PATHFINDER_PROVISIONAL_NODE_LIMIT: usize = 64;

src/entities/shared_components/ambulatory.rs

@@ -1,4 +1,5 @@
 use bevy::prelude::*;
+use bevy::tasks::Task;
 
 #[derive(Component)]
 pub struct Ambulatory {
@@ -18,6 +19,15 @@ pub struct PendingPath {
     pub request_id: u64,
 }
 
+#[derive(Component)]
+pub struct PendingAsyncPath {
+    pub request_id: u64,
+    pub task: Task<Vec<Vec3>>,
+    pub goal: IVec3,
+    pub provisional_path: Vec<Vec3>,
+    pub provisional_path_index: usize,
+}
+
 #[derive(Resource, Default)]
 pub struct PathRequestCounter {
     pub next_id: u64,

src/entities/shared_systems/async_pathfinding.rs

@@ -0,0 +1,296 @@
+use bevy::prelude::*;
+use bevy::tasks::AsyncComputeTaskPool;
+use rustc_hash::{FxHashMap, FxHashSet};
+use std::collections::BinaryHeap;
+
+use crate::constants::{ITILE_SIZE, PATHFINDER_MAX_NODES};
+use crate::entities::shared_components::{CompletedPaths, PendingAsyncPath};
+use crate::world::tiles::tilemap::StandableBitGrid;
+use crate::world::tiles::TileMap;
+
+pub struct AsyncPathfindingPlugin;
+
+impl Plugin for AsyncPathfindingPlugin {
+    fn build(&self, app: &mut App) {
+        app.insert_resource(AsyncPathCounter::default())
+            .add_systems(PostUpdate, poll_async_paths);
+    }
+}
+
+#[derive(Resource, Default)]
+pub struct AsyncPathCounter {
+    pub next_id: u64,
+}
+
+impl AsyncPathCounter {
+    pub fn next(&mut self) -> u64 {
+        let id = self.next_id;
+        self.next_id += 1;
+        id
+    }
+}
+
+#[derive(Clone, Copy, Eq, PartialEq, Debug)]
+struct AsyncPathNode {
+    position: IVec3,
+    f_score: i32,
+    g_score: i32,
+}
+
+impl Ord for AsyncPathNode {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        other
+            .f_score
+            .cmp(&self.f_score)
+            .then_with(|| other.g_score.cmp(&self.g_score))
+    }
+}
+
+impl PartialOrd for AsyncPathNode {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+const ASYNC_ALLOWED_MOVES: [(i32, i32, i32); 24] = [
+    (-1, 0, 0),
+    (1, 0, 0),
+    (0, -1, 0),
+    (0, 1, 0),
+    (-1, -1, 0),
+    (-1, 1, 0),
+    (1, -1, 0),
+    (1, 1, 0),
+    (-1, 0, 1),
+    (-1, 0, -1),
+    (1, 0, 1),
+    (1, 0, -1),
+    (0, -1, 1),
+    (0, -1, -1),
+    (0, 1, 1),
+    (0, 1, -1),
+    (-1, -1, 1),
+    (-1, -1, -1),
+    (-1, 1, 1),
+    (-1, 1, -1),
+    (1, -1, 1),
+    (1, -1, -1),
+    (1, 1, 1),
+    (1, 1, -1),
+];
+
+pub fn calculate_async_path(bit_grid: StandableBitGrid, start: IVec3, goal: IVec3) -> Vec<Vec3> {
+    let (sbx, sby, sbz) = match bit_grid.to_bit_coords(start) {
+        Some(c) => c,
+        None => return vec![Vec3::new(start.x as f32, start.y as f32, start.z as f32)],
+    };
+    let (gbx, gby, gbz) = match bit_grid.to_bit_coords(goal) {
+        Some(c) => c,
+        None => return vec![Vec3::new(start.x as f32, start.y as f32, start.z as f32)],
+    };
+
+    let mut g_scores: FxHashMap<(u32, u32, u32), i32> = FxHashMap::default();
+    let mut came_from: FxHashMap<(u32, u32, u32), (u32, u32, u32)> = FxHashMap::default();
+    let mut closed_set: FxHashSet<(u32, u32, u32)> = FxHashSet::default();
+    let mut open_set: BinaryHeap<AsyncPathNode> = BinaryHeap::new();
+
+    let h = octile_distance_3d_bit(sbx, sby, sbz, gbx, gby, gbz);
+    open_set.push(AsyncPathNode {
+        position: start,
+        f_score: h,
+        g_score: 0,
+    });
+    g_scores.insert((sbx, sby, sbz), 0);
+
+    let mut nodes_expanded: usize = 0;
+
+    while let Some(current_node) = open_set.pop() {
+        let current = current_node.position;
+        nodes_expanded += 1;
+
+        if nodes_expanded > PATHFINDER_MAX_NODES {
+            return reconstruct_path_async(&came_from, current, &bit_grid);
+        }
+
+        if current == goal {
+            return reconstruct_path_async(&came_from, current, &bit_grid);
+        }
+
+        let (cx, cy, cz) = match bit_grid.to_bit_coords(current) {
+            Some(c) => c,
+            None => continue,
+        };
+
+        closed_set.insert((cx, cy, cz));
+
+        for &(dx, dy, dz) in &ASYNC_ALLOWED_MOVES {
+            let nx = cx as i32 + dx;
+            let ny = cy as i32 + dy;
+            let nz = cz as i32 + dz;
+
+            if nx < 0 || ny < 0 || nz < 0 {
+                continue;
+            }
+            let neighbor_bx = nx as u32;
+            let neighbor_by = ny as u32;
+            let neighbor_bz = nz as u32;
+
+            if !bit_grid.is_standable_at(neighbor_bx, neighbor_by, neighbor_bz)
+                || closed_set.contains(&(neighbor_bx, neighbor_by, neighbor_bz))
+            {
+                continue;
+            }
+
+            let movement_cost = calculate_movement_cost_bit(dx, dy, dz);
+            if movement_cost == 0 {
+                continue;
+            }
+
+            let neighbor_pos = IVec3::new(
+                current.x + dx * ITILE_SIZE,
+                current.y + dy * ITILE_SIZE,
+                current.z + dz * ITILE_SIZE,
+            );
+
+            let current_g = *g_scores.get(&(cx, cy, cz)).unwrap_or(&i32::MAX);
+            let new_g = current_g + movement_cost;
+
+            let existing_g = *g_scores
+                .get(&(neighbor_bx, neighbor_by, neighbor_bz))
+                .unwrap_or(&i32::MAX);
+
+            if new_g < existing_g {
+                came_from.insert((neighbor_bx, neighbor_by, neighbor_bz), (cx, cy, cz));
+                g_scores.insert((neighbor_bx, neighbor_by, neighbor_bz), new_g);
+                let h =
+                    octile_distance_3d_bit(neighbor_bx, neighbor_by, neighbor_bz, gbx, gby, gbz);
+                open_set.push(AsyncPathNode {
+                    position: neighbor_pos,
+                    f_score: new_g + h,
+                    g_score: new_g,
+                });
+            }
+        }
+    }
+
+    Vec::new()
+}
+
+fn octile_distance_3d_bit(ax: u32, ay: u32, az: u32, bx: u32, by: u32, bz: u32) -> i32 {
+    let dx = (ax as i32 - bx as i32).abs();
+    let dy = (ay as i32 - by as i32).abs();
+    let dz = (az as i32 - bz as i32).abs();
+    10 * dx.max(dy).max(dz) + 4 * sort_middle(dx, dy, dz) + sort_min(dx, dy, dz)
+}
+
+fn sort_middle(a: i32, b: i32, c: i32) -> i32 {
+    let mut arr = [a, b, c];
+    arr.sort_unstable();
+    arr[1]
+}
+
+fn sort_min(a: i32, b: i32, c: i32) -> i32 {
+    let mut arr = [a, b, c];
+    arr.sort_unstable();
+    arr[0]
+}
+
+fn calculate_movement_cost_bit(dx: i32, dy: i32, dz: i32) -> i32 {
+    match (dx.abs(), dy.abs(), dz.abs()) {
+        (1, 0, 0) | (0, 1, 0) => 10,
+        (1, 1, 0) => 14,
+        (1, 0, 1) | (0, 1, 1) => 42,
+        (1, 1, 1) => 56,
+        _ => 0,
+    }
+}
+
+fn reconstruct_path_async(
+    came_from: &FxHashMap<(u32, u32, u32), (u32, u32, u32)>,
+    mut current: IVec3,
+    bit_grid: &StandableBitGrid,
+) -> Vec<Vec3> {
+    let mut path = vec![Vec3::new(
+        current.x as f32,
+        current.y as f32,
+        current.z as f32,
+    )];
+
+    while let Some((cx, cy, cz)) = bit_grid.to_bit_coords(current) {
+        if let Some(&(px, py, pz)) = came_from.get(&(cx, cy, cz)) {
+            let prev = IVec3::new(
+                bit_grid.origin.x + (px as i32) * ITILE_SIZE,
+                bit_grid.origin.y + (py as i32) * ITILE_SIZE,
+                bit_grid.origin.z + (pz as i32) * ITILE_SIZE,
+            );
+            path.push(Vec3::new(prev.x as f32, prev.y as f32, prev.z as f32));
+            current = prev;
+        } else {
+            break;
+        }
+    }
+
+    path.reverse();
+    path
+}
+
+pub fn poll_async_paths(
+    mut commands: Commands,
+    mut pending_query: Query<(Entity, &mut PendingAsyncPath)>,
+    mut completed: ResMut<CompletedPaths>,
+) {
+    for (entity, mut pending) in pending_query.iter_mut() {
+        if !pending.task.is_finished() {
+            continue;
+        }
+        let path: Option<Vec<Vec3>> =
+            futures_lite::future::block_on(futures_lite::future::poll_once(&mut pending.task));
+        if let Some(p) = path {
+            if !p.is_empty() {
+                completed.paths.push((pending.request_id, p));
+            }
+            commands.entity(entity).remove::<PendingAsyncPath>();
+        }
+    }
+}
+
+pub fn compute_bounding_box(start: IVec3, goal: IVec3, margin_tiles: i32) -> (IVec3, UVec3) {
+    let margin = margin_tiles * ITILE_SIZE;
+    let min_x = start.x.min(goal.x) - margin;
+    let max_x = start.x.max(goal.x) + margin;
+    let min_y = start.y.min(goal.y) - margin;
+    let max_y = start.y.max(goal.y) + margin;
+    let min_z = start.z.min(goal.z) - margin;
+    let max_z = start.z.max(goal.z) + margin;
+
+    let origin = IVec3::new(min_x, min_y, min_z);
+    let size = UVec3::new(
+        ((max_x - min_x) / ITILE_SIZE + 1) as u32,
+        ((max_y - min_y) / ITILE_SIZE + 1) as u32,
+        ((max_z - min_z) / ITILE_SIZE + 1) as u32,
+    );
+
+    (origin, size)
+}
+
+pub fn spawn_async_path_task(
+    tilemap: &TileMap,
+    start: IVec3,
+    goal: IVec3,
+    provisional_path: Vec<Vec3>,
+    request_id: u64,
+) -> PendingAsyncPath {
+    let (origin, size) = compute_bounding_box(start, goal, 20);
+    let bit_grid = StandableBitGrid::new(origin, size, tilemap);
+
+    let pool = AsyncComputeTaskPool::get();
+    let task = pool.spawn(async move { calculate_async_path(bit_grid, start, goal) });
+
+    PendingAsyncPath {
+        request_id,
+        task,
+        goal,
+        provisional_path,
+        provisional_path_index: 0,
+    }
+}

src/entities/shared_systems/mod.rs

@@ -1 +1,2 @@
+pub mod async_pathfinding;
 pub mod pathfinding;

src/entities/shared_systems/pathfinding.rs

@@ -1,10 +1,15 @@
 use bevy::prelude::*;
+use bevy::tasks::AsyncComputeTaskPool;
 use rayon::prelude::*;
 use rustc_hash::FxHashMap;
 use rustc_hash::FxHashSet;
 use std::{cell::RefCell, collections::BinaryHeap, time::Instant};
 
-use crate::constants::{ITILE_SIZE, PATHFINDER_MAX_NODES, TILE_SIZE};
+use crate::constants::{
+    ITILE_SIZE, PATHFINDER_MAX_NODES, PATHFINDER_PROVISIONAL_NODE_LIMIT, TILE_SIZE,
+};
+use crate::entities::shared_components::PendingAsyncPath;
+use crate::world::tiles::tilemap::StandableBitGrid;
 use crate::world::tiles::TileMap;
 use crate::world::{chunks::ChunkMap, chunks::CHUNK_SIZE};
 use crate::{constants::*, entities::shared_components::Ambulatory};
@@ -136,15 +141,116 @@ impl Plugin for PathfindingPlugin {
         app.insert_resource(PathfindingBenchmark::new(100))
             .insert_resource(crate::entities::shared_components::CompletedPaths::default())
             .insert_resource(crate::entities::shared_components::PathRequestCounter::default())
-            .add_systems(FixedUpdate, (update_wandering_targets, movement).chain())
+            .insert_resource(
+                crate::entities::shared_systems::async_pathfinding::AsyncPathCounter::default(),
+            )
+            .add_systems(
+                FixedUpdate,
+                (prepare_paths, update_wandering_targets, movement).chain(),
+            )
             .add_systems(
                 PostUpdate,
-                (merge_benchmark_stats, process_completed_paths).chain(),
+                (
+                    merge_benchmark_stats,
+                    process_completed_paths,
+                    crate::entities::shared_systems::async_pathfinding::poll_async_paths,
+                    splice_completed_async_paths,
+                ),
             )
             .add_systems(Update, bench_report_system);
     }
 }
 
+pub fn prepare_paths(
+    mut commands: Commands,
+    mut counter: ResMut<crate::entities::shared_systems::async_pathfinding::AsyncPathCounter>,
+    mut query: Query<
+        (
+            Entity,
+            &mut crate::entities::shared_components::Ambulatory,
+            &Transform,
+            Option<&crate::entities::shared_components::PendingAsyncPath>,
+        ),
+        Without<crate::entities::shared_components::PendingPath>,
+    >,
+    tilemap: Res<TileMap>,
+) {
+    for (entity, mut ambulatory, transform, pending_async) in query.iter_mut() {
+        if let Some(target) = ambulatory.target {
+            if ambulatory.current_path.is_none() || pending_async.is_some() {
+                continue;
+            }
+            let start = transform.translation.as_ivec3();
+            let goal = target.as_ivec3() - ivec3(0, 0, ITILE_SIZE);
+            let distance = octile_distance_3d(start, goal) / ITILE_SIZE;
+
+            if distance <= PATHFINDER_SHORT_PATH_MAX_TILES {
+                let path = calculate_path_benchmarked(&tilemap, start, goal);
+                ambulatory.current_path = Some(path);
+                ambulatory.path_index = 0;
+            } else {
+                let provisional = calculate_provisional_path(
+                    &tilemap,
+                    start,
+                    goal,
+                    PATHFINDER_PROVISIONAL_NODE_LIMIT,
+                );
+                if !provisional.is_empty() {
+                    ambulatory.current_path = Some(provisional.clone());
+                    ambulatory.path_index = 0;
+
+                    let request_id = counter.next();
+                    let pending =
+                        crate::entities::shared_systems::async_pathfinding::spawn_async_path_task(
+                            &tilemap,
+                            start,
+                            goal,
+                            provisional,
+                            request_id,
+                        );
+                    commands.entity(entity).insert(pending);
+                } else {
+                    let path = calculate_path_benchmarked(&tilemap, start, goal);
+                    ambulatory.current_path = Some(path);
+                    ambulatory.path_index = 0;
+                }
+            }
+        }
+    }
+}
+
+pub fn splice_completed_async_paths(
+    mut completed: ResMut<crate::entities::shared_components::CompletedPaths>,
+    mut query: Query<(
+        Entity,
+        &mut crate::entities::shared_components::Ambulatory,
+        &Transform,
+    )>,
+) {
+    if completed.paths.is_empty() {
+        return;
+    }
+
+    let mut to_process = Vec::new();
+    for (request_id, path) in completed.paths.drain(..) {
+        to_process.push((request_id, path));
+    }
+
+    for (request_id, path) in to_process {
+        for (entity, mut ambulatory, _transform) in query.iter_mut() {
+            if ambulatory.current_path.as_ref().is_none_or(|p| p != &path) {
+                let splice_index = find_splice_point(&path, ambulatory.path_index);
+                ambulatory.current_path = Some(path.clone());
+                ambulatory.path_index = splice_index;
+            }
+        }
+    }
+}
+
+fn find_splice_point(path: &[Vec3], current_index: usize) -> usize {
+    current_index.min(path.len().saturating_sub(1))
+}
+
 pub fn process_completed_paths(
     mut completed: ResMut<crate::entities::shared_components::CompletedPaths>,
     mut query: Query<(
@@ -249,43 +355,37 @@ pub fn movement(mut query: Query<(&mut Ambulatory, &mut Transform)>, tilemap: Re
                 return;
             }
 
-            if let Some(target) = ambulatory.target {
-                if ambulatory.current_path.is_none() {
-                    ambulatory.current_path = Some(calculate_path_benchmarked(
-                        &tilemap,
-                        transform.translation.as_ivec3(),
-                        target.as_ivec3() - ivec3(0, 0, 1),
-                    ));
-                    ambulatory.path_index = 0;
+            if ambulatory.current_path.is_none() {
+                return;
+            }
+
+            if ambulatory.walk_speed > 0. {
+                if ambulatory.step_recovery <= ambulatory.walk_speed as u32 {
+                    ambulatory.step_recovery += 1;
+                    return;
+                } else {
+                    ambulatory.step_recovery = 0;
                 }
-                if ambulatory.walk_speed > 0. {
-                    if ambulatory.step_recovery <= ambulatory.walk_speed as u32 {
-                        ambulatory.step_recovery += 1;
-                        return;
-                    } else {
-                        ambulatory.step_recovery = 0;
+            }
+
+            if let Some(path) = &ambulatory.current_path {
+                if ambulatory.path_index < path.len() {
+                    let next_point = path[ambulatory.path_index];
+                    let direction = (next_point - transform.translation).normalize();
+                    transform.translation = next_point;
+
+                    if direction.x > 0.0 {
+                        transform.scale.x = PIXEL_RATIO;
+                    } else if direction.x < 0.0 {
+                        transform.scale.x = -PIXEL_RATIO;
                     }
-                }
 
-                if let Some(path) = &ambulatory.current_path {
-                    if ambulatory.path_index < path.len() {
-                        let next_point = path[ambulatory.path_index];
-                        let direction = (next_point - transform.translation).normalize();
-                        transform.translation = next_point;
-
-                        if direction.x > 0.0 {
-                            transform.scale.x = PIXEL_RATIO;
-                        } else if direction.x < 0.0 {
-                            transform.scale.x = -PIXEL_RATIO;
-                        }
-
-                        if transform.translation.distance(next_point) < TILE_SIZE {
-                            ambulatory.path_index += 1;
-                        }
-                    } else {
-                        ambulatory.current_path = None;
-                        ambulatory.target = None;
+                    if transform.translation.distance(next_point) < TILE_SIZE {
+                        ambulatory.path_index += 1;
                     }
+                } else {
+                    ambulatory.current_path = None;
+                    ambulatory.target = None;
                 }
             }
         });
@@ -466,6 +566,88 @@ fn calculate_path_with_scratchpad(
     })
 }
 
+pub fn calculate_provisional_path(
+    tilemap: &TileMap,
+    start: IVec3,
+    goal: IVec3,
+    node_limit: usize,
+) -> Vec<Vec3> {
+    if !is_standable_tile(tilemap, start) {
+        return Vec::new();
+    }
+
+    let estimated_tiles = octile_distance_3d(start, goal) / ITILE_SIZE;
+
+    SCRATCHPAD.with(|s| {
+        let mut scratch = s.borrow_mut();
+        let capacity = ((estimated_tiles as usize).max(64)).min(4096);
+        scratch.clear_and_reserve(capacity);
+
+        let initial_h = octile_distance_3d(start, goal);
+        scratch.open_set.push(PathNode {
+            position: start,
+            f_score: initial_h,
+            g_score: 0,
+        });
+        scratch.g_scores.insert(start, 0);
+
+        let mut nodes_expanded: usize = 0;
+        let mut best_node = start;
+        let mut best_h = initial_h;
+
+        while let Some(current_node) = scratch.open_set.pop() {
+            let current = current_node.position;
+            nodes_expanded += 1;
+
+            let h = octile_distance_3d(current, goal);
+            if h < best_h {
+                best_h = h;
+                best_node = current;
+            }
+
+            if current == goal {
+                return reconstruct_path(&scratch.came_from, current);
+            }
+
+            if nodes_expanded >= node_limit {
+                return reconstruct_path(&scratch.came_from, best_node);
+            }
+
+            scratch.closed_set.insert(current);
+
+            for &move_dir in &ALLOWED_MOVES {
+                let neighbor_pos = current + move_dir;
+
+                if !is_standable_tile(tilemap, neighbor_pos)
+                    || scratch.closed_set.contains(&neighbor_pos)
+                {
+                    continue;
+                }
+
+                let movement_cost = calculate_movement_cost(move_dir);
+                if movement_cost == 0 {
+                    continue;
+                }
+
+                let new_g = *scratch.g_scores.get(&current).unwrap_or(&i32::MAX) + movement_cost;
+
+                if new_g < *scratch.g_scores.get(&neighbor_pos).unwrap_or(&i32::MAX) {
+                    scratch.came_from.insert(neighbor_pos, current);
+                    scratch.g_scores.insert(neighbor_pos, new_g);
+                    let f = new_g + octile_distance_3d(neighbor_pos, goal);
+                    scratch.open_set.push(PathNode {
+                        position: neighbor_pos,
+                        f_score: f,
+                        g_score: new_g,
+                    });
+                }
+            }
+        }
+
+        reconstruct_path(&scratch.came_from, best_node)
+    })
+}
+
 pub fn bench_report_system(
     keys: Res<ButtonInput<KeyCode>>,
     mut bench: ResMut<PathfindingBenchmark>,

src/world/tiles/tilemap.rs

@@ -1,6 +1,8 @@
 use bevy::prelude::*;
 use rustc_hash::FxHashMap;
 
+use crate::constants::ITILE_SIZE;
+
 /// Packed floor tile data for efficient storage. ~35 bytes vs 76 bytes tuple.
 #[derive(Clone, Copy, Debug)]
 pub struct FloorTileData {
@@ -193,3 +195,98 @@ impl TileMap {
         self.floor_tiles.get_mut(pos)
     }
 }
+
+/// Bit-packed bounding-box snapshot for async pathfinding.
+/// 1 bit per tile = ~6KB for 50,000 tiles vs HashMap overhead.
+/// Must be Send+Sync — no RefCell, no Arc.
+#[derive(Clone, Debug)]
+pub struct StandableBitGrid {
+    pub origin: IVec3,
+    pub size: UVec3,
+    pub bits: Vec<u64>,
+}
+
+impl StandableBitGrid {
+    /// Create a bit-grid snapshot of all standable tiles within bounding box.
+    /// origin: min corner (inclusive), snapped to ITILE_SIZE
+    /// size: dimensions in tiles (not pixels)
+    pub fn new(origin: IVec3, size: UVec3, tilemap: &TileMap) -> Self {
+        let total_bits = (size.x * size.y * size.z) as usize;
+        let words = (total_bits + 63) / 64;
+        let mut bits = vec![0u64; words];
+
+        for bz in 0..size.z {
+            for by in 0..size.y {
+                for bx in 0..size.x {
+                    let pos = IVec3::new(
+                        origin.x + (bx as i32) * ITILE_SIZE,
+                        origin.y + (by as i32) * ITILE_SIZE,
+                        origin.z + (bz as i32) * ITILE_SIZE,
+                    );
+                    if Self::tile_is_standable(tilemap, pos) {
+                        let idx = ((bz * size.y * size.x) + (by * size.x) + bx) as usize;
+                        bits[idx / 64] |= 1u64 << (idx % 64);
+                    }
+                }
+            }
+        }
+
+        Self { origin, size, bits }
+    }
+
+    /// Standable check using TileMap (mirrors pathfinding.rs::is_standable_tile)
+    #[inline]
+    fn tile_is_standable(tilemap: &TileMap, pos: IVec3) -> bool {
+        let can_stand_in_tile = tilemap
+            .floor_tiles
+            .get(&pos)
+            .map(|t| t.can_stand_in())
+            .unwrap_or(false);
+        let can_stand_in_fixture = tilemap
+            .fixture_tiles
+            .get(&pos)
+            .map(|t| t.can_stand_in())
+            .unwrap_or(false);
+        let pos_below = pos - IVec3::new(0, 0, ITILE_SIZE);
+        let can_stand_on_tile_below = tilemap
+            .floor_tiles
+            .get(&pos_below)
+            .map(|t| t.can_stand_on())
+            .unwrap_or(false);
+        let can_stand_on_fixture_below = tilemap
+            .fixture_tiles
+            .get(&pos_below)
+            .map(|t| t.can_stand_on())
+            .unwrap_or(false);
+
+        (can_stand_in_tile || can_stand_in_fixture)
+            && (can_stand_on_tile_below || can_stand_on_fixture_below)
+    }
+
+    /// O(1) standable check using bit-grid coordinates.
+    #[inline]
+    pub fn is_standable_at(&self, bx: u32, by: u32, bz: u32) -> bool {
+        if bx >= self.size.x || by >= self.size.y || bz >= self.size.z {
+            return false;
+        }
+        let idx = ((bz * self.size.y * self.size.x) + (by * self.size.x) + bx) as usize;
+        self.bits[idx / 64] & (1u64 << (idx % 64)) != 0
+    }
+
+    /// Convert IVec3 world position to bit-grid coordinates.
+    /// Returns None if position is outside the grid bounds.
+    #[inline]
+    pub fn to_bit_coords(&self, pos: IVec3) -> Option<(u32, u32, u32)> {
+        let local = pos - self.origin;
+        if local.x < 0 || local.y < 0 || local.z < 0 {
+            return None;
+        }
+        let bx = (local.x / ITILE_SIZE) as u32;
+        let by = (local.y / ITILE_SIZE) as u32;
+        let bz = (local.z / ITILE_SIZE) as u32;
+        if bx >= self.size.x || by >= self.size.y || bz >= self.size.z {
+            return None;
+        }
+        Some((bx, by, bz))
+    }
+}