✨ 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/types.ts

@@ -20,7 +20,10 @@ export const IMPASSABLE = new Set<number>([
   TileType.WALL,
 ])
 
-export type ItemId = 'wood' | 'stone' | 'wheat_seed' | 'carrot_seed' | 'wheat' | 'carrot'
+/** Tiles on which tree seedlings may be planted. */
+export const PLANTABLE_TILES = new Set<TileType>([TileType.GRASS, TileType.DARK_GRASS])
+
+export type ItemId = 'wood' | 'stone' | 'wheat_seed' | 'carrot_seed' | 'wheat' | 'carrot' | 'tree_seed'
 
 export type BuildingType = 'floor' | 'wall' | 'chest' | 'bed' | 'stockpile_zone'
 
@@ -90,6 +93,18 @@ export interface PlayerState {
   inventory: Partial<Record<ItemId, number>>
 }
 
+export interface TreeSeedlingState {
+  id: string
+  tileX: number
+  tileY: number
+  /** Growth stage: 0 = sprout, 1 = sapling, 2 = mature (converts to resource). */
+  stage: number
+  /** Time remaining until next stage advance, in milliseconds. */
+  stageTimerMs: number
+  /** The tile type that was under the seedling when planted (GRASS or DARK_GRASS). */
+  underlyingTile: TileType
+}
+
 export interface WorldState {
   seed: number
   tiles: number[]
@@ -98,6 +113,13 @@ export interface WorldState {
   crops: Record<string, CropState>
   villagers: Record<string, VillagerState>
   stockpile: Partial<Record<ItemId, number>>
+  /** Planted tree seedlings, keyed by ID. */
+  treeSeedlings: Record<string, TreeSeedlingState>
+  /**
+   * Recovery timers for DARK_GRASS tiles, keyed by "tileX,tileY".
+   * Value is remaining milliseconds until the tile reverts to GRASS.
+   */
+  tileRecovery: Record<string, number>
 }
 
 export interface GameStateData {
@@ -123,3 +145,7 @@ export type GameAction =
   | { type: 'VILLAGER_HARVEST_CROP';    villagerId: string; cropId: string }
   | { type: 'VILLAGER_DEPOSIT';         villagerId: string }
   | { type: 'UPDATE_PRIORITIES';        villagerId: string; priorities: JobPriorities }
+  | { type: 'PLANT_TREE_SEED';         seedling: TreeSeedlingState }
+  | { type: 'REMOVE_TREE_SEEDLING';    seedlingId: string }
+  | { type: 'SPAWN_RESOURCE';          resource: ResourceNodeState }
+  | { type: 'TILE_RECOVERY_START';     tileX: number; tileY: number }