✨ 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/ResourceSystem.ts

@@ -76,6 +76,16 @@ export class ResourceSystem {
     this.removeSprite(id)
   }
 
+  /**
+   * Spawns a sprite for a resource that was created at runtime
+   * (e.g. a tree grown from a seedling). The resource must already be
+   * present in the game state when this is called.
+   * @param node - The resource node to render
+   */
+  public spawnResourcePublic(node: ResourceNodeState): void {
+    this.spawnSprite(node)
+  }
+
   /** Called when WorldSystem changes a tile (e.g. after tree removed) */
   syncTileChange(tileX: number, tileY: number, worldSystem: { setTile: (x: number, y: number, type: TileType) => void }): void {
     const state = stateManager.getState()