♻️ 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/scenes/GameScene.ts

@@ -80,6 +80,7 @@ export class GameScene extends Phaser.Scene {
         this.worldSystem.setTile(action.tileX, action.tileY, action.tile)
       } else if (action.type === 'SPAWN_RESOURCE') {
         this.resourceSystem.spawnResourcePublic(action.resource)
+        this.worldSystem.addResourceTile(action.resource.tileX, action.resource.tileY)
       }
     }