♻️ 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/systems/VillagerSystem.ts

@@ -274,10 +274,9 @@ export class VillagerSystem {
       const res = state.world.resources[job.targetId]
       if (res) {
         this.adapter.send({ type: 'VILLAGER_HARVEST_RESOURCE', villagerId: v.id, resourceId: job.targetId })
-        // Clear the FOREST tile so the area becomes passable for future pathfinding
         this.adapter.send({ type: 'CHANGE_TILE', tileX: res.tileX, tileY: res.tileY, tile: TileType.DARK_GRASS })
-        // Start recovery timer so DARK_GRASS reverts to GRASS after 5 minutes
         this.adapter.send({ type: 'TILE_RECOVERY_START', tileX: res.tileX, tileY: res.tileY })
+        this.worldSystem.removeResourceTile(res.tileX, res.tileY)
         this.resourceSystem.removeResource(job.targetId)
         this.addLog(v.id, '✓ Chopped tree (+2 wood)')
       }
@@ -285,8 +284,8 @@ export class VillagerSystem {
       const res = state.world.resources[job.targetId]
       if (res) {
         this.adapter.send({ type: 'VILLAGER_HARVEST_RESOURCE', villagerId: v.id, resourceId: job.targetId })
-        // Clear the ROCK tile so the area becomes passable for future pathfinding
         this.adapter.send({ type: 'CHANGE_TILE', tileX: res.tileX, tileY: res.tileY, tile: TileType.GRASS })
+        this.worldSystem.removeResourceTile(res.tileX, res.tileY)
         this.resourceSystem.removeResource(job.targetId)
         this.addLog(v.id, '✓ Mined rock (+2 stone)')
       }