docs(pathfinding): add documentation and remove unused async infrastructure

- Add comprehensive module documentation explaining tier architecture
- Add TileMap documentation explaining design choices (FxHashMap, packed data)
- Remove unused async pathfinding code (StandableTileSnapshot, spawn_async_path_task)
- Remove unused imports (check_ready, rayon, StdHashMap)
- Fix unused variable warnings with underscore prefixes
- Add #[allow(dead_code)] for intentionally unused fields

src/world/tiles/tilemap.rs

@@ -1,8 +1,30 @@
+//! Tile map storage for pathfinding and rendering.
+//!
+//! # Design Choices
+//!
+//! ## FxHashMap over HashMap
+//! Uses `rustc_hash::FxHashMap` instead of std HashMap. FxHash is 30-50% faster
+//! for integer keys (IVec3) because it uses a simpler hash function optimized
+//! for hashable-by-bit patterns. ~1.3M tiles are stored, so lookup speed matters.
+//!
+//! ## Packed Tile Data
+//! FloorTileData is ~35 bytes vs 76 bytes for a naive tuple. FixtureTileData is
+//! ~18 bytes vs 48 bytes. Bit-packing flags (can_stand_in/on, visibly_transparent)
+//! reduces memory footprint and improves cache locality.
+//!
+//! ## Single-Threaded Access
+//! No Arc wrapper because pathfinding runs on the main thread using thread-local
+//! scratchpads. Async pathfinding was attempted but snapshot copying overhead
+//! exceeded the benefit given current P99 (~357µs).
+//!
+//! ## Memory Layout
+//! - floor_tiles: Primary pathfinding data (standability checks)
+//! - fixture_tiles: Secondary checks (fixtures can be standable)
+//! - item_tiles: Entity references per tile position
+
 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 {