♻️ resource-based passability: FOREST/ROCK walkable without a resource (Issue #22)

Previously FOREST and ROCK tile types were always impassable, making 30 % of
forest floor and 50 % of rocky terrain permanently blocked even with no object
on them.

- Remove FOREST + ROCK from IMPASSABLE in types.ts
- Add RESOURCE_TERRAIN set (FOREST, ROCK) for tiles that need resource check
- WorldSystem: add resourceTiles Set<number> as O(1) spatial index
  - initResourceTiles() builds index from state on create()
  - addResourceTile() / removeResourceTile() keep it in sync at runtime
- isPassable() now: impassable tiles → false | RESOURCE_TERRAIN → check index | else → true
- GameScene: call addResourceTile() when SPAWN_RESOURCE fires (seedling matures)
- VillagerSystem: call removeResourceTile() after chop / mine completes

Side effect: trees fully enclosed by other trees are now reachable once an
adjacent tree is cleared; the hasAdjacentPassable() guard in pickJob still
correctly skips resources with zero passable neighbours.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/types.ts

@@ -12,14 +12,21 @@ export enum TileType {
   WATERED_SOIL  = 10,
 }
 
+/** Tiles that are always impassable regardless of what is on them. */
 export const IMPASSABLE = new Set<number>([
   TileType.DEEP_WATER,
   TileType.SHALLOW_WATER,
-  TileType.FOREST,
-  TileType.ROCK,
   TileType.WALL,
 ])
 
+/**
+ * Terrain tiles whose passability depends on whether a resource
+ * (tree or rock) is currently placed on them.
+ * An empty FOREST tile is walkable forest floor; a ROCK tile without a
+ * rock resource is just rocky ground.
+ */
+export const RESOURCE_TERRAIN = new Set<number>([TileType.FOREST, TileType.ROCK])
+
 /** Tiles on which tree seedlings may be planted. */
 export const PLANTABLE_TILES = new Set<TileType>([TileType.GRASS, TileType.DARK_GRASS])