✨ 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/WorldSystem.ts

@@ -21,6 +21,7 @@ export class WorldSystem {
   private tileset!: Phaser.Tilemaps.Tileset
   private bgImage!: Phaser.GameObjects.Image
   private builtLayer!: Phaser.Tilemaps.TilemapLayer
+  private bgCanvasTexture!: Phaser.Textures.CanvasTexture
 
   /** @param scene - The Phaser scene this system belongs to */
   constructor(scene: Phaser.Scene) {
@@ -35,10 +36,8 @@ export class WorldSystem {
     const state = stateManager.getState()
 
     // --- Canvas background (1px per tile, scaled up, LINEAR filtered) ---
-    const canvas = document.createElement('canvas')
-    canvas.width = WORLD_TILES
-    canvas.height = WORLD_TILES
-    const ctx = canvas.getContext('2d')!
+    const canvasTexture = this.scene.textures.createCanvas('terrain_bg', WORLD_TILES, WORLD_TILES) as Phaser.Textures.CanvasTexture
+    const ctx = canvasTexture.context
 
     for (let y = 0; y < WORLD_TILES; y++) {
       for (let x = 0; x < WORLD_TILES; x++) {
@@ -48,12 +47,14 @@ export class WorldSystem {
       }
     }
 
-    this.scene.textures.addCanvas('terrain_bg', canvas)
+    canvasTexture.refresh()
+    this.bgCanvasTexture = canvasTexture
+
     this.bgImage = this.scene.add.image(0, 0, 'terrain_bg')
       .setOrigin(0, 0)
       .setScale(TILE_SIZE)
       .setDepth(0)
-    this.scene.textures.get('terrain_bg').setFilter(Phaser.Textures.FilterMode.LINEAR)
+    canvasTexture.setFilter(Phaser.Textures.FilterMode.LINEAR)
 
     // --- Built tile layer (sparse — only FLOOR, WALL, TILLED_SOIL, WATERED_SOIL) ---
     this.map = this.scene.make.tilemap({
@@ -157,6 +158,21 @@ export class WorldSystem {
     return state.world.tiles[tileY * WORLD_TILES + tileX] as TileType
   }
 
+  /**
+   * Updates a single tile's pixel on the background canvas and refreshes the GPU texture.
+   * Used when a natural tile changes at runtime (e.g. DARK_GRASS → GRASS after recovery,
+   * or GRASS → FOREST when a seedling matures).
+   * @param tileX - Tile column
+   * @param tileY - Tile row
+   * @param type  - New tile type to reflect visually
+   */
+  refreshTerrainTile(tileX: number, tileY: number, type: TileType): void {
+    const color = BIOME_COLORS[type] ?? '#0a2210'
+    this.bgCanvasTexture.context.fillStyle = color
+    this.bgCanvasTexture.context.fillRect(tileX, tileY, 1, 1)
+    this.bgCanvasTexture.refresh()
+  }
+
   /** Destroys the tilemap and background image. */
   destroy(): void {
     this.map.destroy()