✨ implement unified tile system (Issue #14)

- Tree seedlings: plant tree_seed on grass via farming tool; two-stage
  growth (sprout → sapling → young tree, ~1 min/stage); matures into
  a harvestable FOREST resource tile
- Tile recovery: Nisse chops start a 5-min DARK_GRASS→GRASS timer;
  terrain canvas updated live via WorldSystem.refreshTerrainTile()
- New TreeSeedlingSystem manages sprites, growth ticking, maturation
- BootScene generates seedling_0/1/2 textures procedurally
- FarmingSystem adds tree_seed to tool cycle (F key)
- Stockpile panel shows tree_seed (default: 5); panel height adjusted
- StateManager v5: treeSeedlings + tileRecovery in WorldState
- WorldSystem uses CanvasTexture for live single-pixel updates

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

src/systems/VillagerSystem.ts

@@ -276,6 +276,8 @@ export class VillagerSystem {
         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.resourceSystem.removeResource(job.targetId)
         this.addLog(v.id, '✓ Chopped tree (+2 wood)')
       }