fix: snap-to-top for above-world spawn, guard rabbit dig at world floor

- Entities spawn at z=35*TILE=560 but Z_ABOVE max is 15*TILE=240. Snap to
  Z_ABOVE*TILE instead of falling one tile per tick through unloaded space.
- Skip rabbit dig when below_pos would be outside ChunkData z-bounds,
  preventing remove_floor assert panic.

src/entities/shared_systems/pathfinding.rs

@@ -68,7 +68,7 @@ use crate::world::tiles::tile_changed::{PathfindingDirtyChunks, TileChangedEvent
 use crate::world::tiles::TileMap;
 use crate::world::{
     chunks::CHUNK_SIZE,
-    chunks::{world_to_chunk, ChunkMap},
+    chunks::{world_to_chunk, ChunkMap, Z_ABOVE, Z_BELOW},
 };
 use crate::{constants::*, entities::shared_components::Ambulatory};
 use bevy::math::ivec3;
@@ -609,10 +609,12 @@ pub fn movement(
         .for_each(|(mut ambulatory, mut transform)| {
             let current_pos = transform.translation;
             if !is_standable_tile(&tilemap, current_pos.as_ivec3()) {
-                // Don't fall below world minimum z — rabbits digging can leave no floor
-                // to stand on, but they should stop at the world floor rather than
-                // falling into oblivion and panicking pos_to_index overflow.
-                if transform.translation.z > -(crate::world::chunks::Z_BELOW as f32) * TILE_SIZE {
+                // Entities can spawn above the loaded world range (z > Z_ABOVE*TILE_SIZE).
+                // Snap them to the top instead of falling through unloaded z-space one
+                // tile per tick.
+                if transform.translation.z > Z_ABOVE as f32 * TILE_SIZE {
+                    transform.translation.z = Z_ABOVE as f32 * TILE_SIZE;
+                } else if transform.translation.z > -(Z_BELOW as f32) * TILE_SIZE {
                     transform.translation.z -= TILE_SIZE;
                 }
                 ambulatory.current_path = None;