feat: soft entity collision, dead code removal, leaf standability fix

- Add TileOccupancy resource and rebuild_tile_occupancy system to track
  per-tile entity counts for soft collision avoidance
- Add collision_delay to Ambulatory; entities try relative-left step on
  occupied tiles, wait 1 tick, then push through
- Fix leaf canopy standability: leaves are now can_stand_in=true,
  can_stand_on=false (walkable, not standable-on)
- Delete FloorTilePrefab / FixtureTilePrefab / FloorTile / FixtureTile /
  TileState (all fully dead — TileMap + ChunkData are sole truth)
- Delete tile_spawns from TerrainBlob (populated but never consumed)
- Delete leaf ghost entity spawn in forestry (orphaned invisible ECS entity)
- Replace log prefab spawn with inline commands.spawn(Transform, Visibility)
- Add TileMap::remove_fixture for future digging/explosion use
- Skip collision avoidance when current tile has >2 entities (handles spawn
  cluster deadlock)

src/world/tiles/tilemap.rs

@@ -237,6 +237,24 @@ impl TileMap {
             .unwrap_or(100)
     }
 
+    /// Remove a fixture tile, clearing both the HashMap entry and ChunkData bitsets.
+    /// BOTH must be cleared — leaving ChunkData stale causes is_standable bugs.
+    pub fn remove_fixture(&mut self, pos: &IVec3) -> Option<FixtureTileData> {
+        use super::chunk_data::ChunkData;
+
+        let removed = self.fixture_tiles.remove(pos);
+        let chunk_pos = world_to_chunk(*pos);
+        if let Some(chunk) = self.chunks.get_mut(&chunk_pos) {
+            let (lx, ly, z) = ChunkData::world_to_local(*pos);
+            let idx = ChunkData::pos_to_index(lx, ly, z);
+            let word = idx / 32;
+            let clear_mask = !(1u32 << (idx % 32));
+            chunk.stand_in_fixture[word] &= clear_mask;
+            chunk.stand_on_fixture[word] &= clear_mask;
+        }
+        removed
+    }
+
     /// Remove all tile data for a specific chunk from the TileMap.
     /// Iterates all positions in the chunk volume and removes from HashMaps.
     /// Used during chunk unloading to clean up tile data.