✨ 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/StateManager.ts

@@ -1,5 +1,6 @@
-import { SAVE_KEY, WORLD_TILES, CROP_CONFIGS } from './config'
+import { SAVE_KEY, WORLD_TILES, CROP_CONFIGS, TREE_SEEDLING_STAGE_MS, TILE_RECOVERY_MS } from './config'
 import type { GameStateData, GameAction, PlayerState, WorldState, ItemId } from './types'
+import { TileType } from './types'
 
 const DEFAULT_PLAYER: PlayerState = {
   id: 'player1',
@@ -15,13 +16,15 @@ function makeEmptyWorld(seed: number): WorldState {
     buildings: {},
     crops: {},
     villagers: {},
-    stockpile: { wood: 10, stone: 5, wheat_seed: 10, carrot_seed: 5, wheat: 0, carrot: 0 },
+    stockpile: { wood: 10, stone: 5, wheat_seed: 10, carrot_seed: 5, wheat: 0, carrot: 0, tree_seed: 5 },
+    treeSeedlings: {},
+    tileRecovery: {},
   }
 }
 
 function makeDefaultState(): GameStateData {
   return {
-    version: 4,
+    version: 5,
     world: makeEmptyWorld(Math.floor(Math.random() * 999999)),
     player: { ...DEFAULT_PLAYER, inventory: { ...DEFAULT_PLAYER.inventory } },
   }
@@ -146,6 +149,26 @@ class StateManager {
         if (v) v.priorities = { ...action.priorities }
         break
       }
+
+      case 'PLANT_TREE_SEED': {
+        w.treeSeedlings[action.seedling.id] = { ...action.seedling }
+        w.stockpile.tree_seed = Math.max(0, (w.stockpile.tree_seed ?? 0) - 1)
+        // Cancel any tile recovery on this tile
+        delete w.tileRecovery[`${action.seedling.tileX},${action.seedling.tileY}`]
+        break
+      }
+
+      case 'REMOVE_TREE_SEEDLING':
+        delete w.treeSeedlings[action.seedlingId]
+        break
+
+      case 'SPAWN_RESOURCE':
+        w.resources[action.resource.id] = { ...action.resource }
+        break
+
+      case 'TILE_RECOVERY_START':
+        w.tileRecovery[`${action.tileX},${action.tileY}`] = TILE_RECOVERY_MS
+        break
     }
   }
 
@@ -163,6 +186,47 @@ class StateManager {
     return advanced
   }
 
+  /**
+   * Advances all tree-seedling growth timers.
+   * Returns IDs of seedlings that have reached stage 2 (ready to mature into a tree).
+   * @param delta - Frame delta in milliseconds
+   * @returns Array of seedling IDs that are now mature
+   */
+  tickSeedlings(delta: number): string[] {
+    const advanced: string[] = []
+    for (const s of Object.values(this.state.world.treeSeedlings)) {
+      s.stageTimerMs -= delta
+      if (s.stageTimerMs <= 0) {
+        s.stage = Math.min(s.stage + 1, 2)
+        s.stageTimerMs = TREE_SEEDLING_STAGE_MS
+        advanced.push(s.id)
+      }
+    }
+    return advanced
+  }
+
+  /**
+   * Ticks tile-recovery timers.
+   * Returns keys ("tileX,tileY") of tiles that have now recovered back to GRASS.
+   * @param delta - Frame delta in milliseconds
+   * @returns Array of recovered tile keys
+   */
+  tickTileRecovery(delta: number): string[] {
+    const recovered: string[] = []
+    const rec = this.state.world.tileRecovery
+    for (const key of Object.keys(rec)) {
+      rec[key] -= delta
+      if (rec[key] <= 0) {
+        delete rec[key]
+        recovered.push(key)
+        // Update tiles array directly (DARK_GRASS → GRASS)
+        const [tx, ty] = key.split(',').map(Number)
+        this.state.world.tiles[ty * WORLD_TILES + tx] = TileType.GRASS
+      }
+    }
+    return recovered
+  }
+
   save(): void {
     try { localStorage.setItem(SAVE_KEY, JSON.stringify(this.state)) } catch (_) {}
   }
@@ -172,10 +236,12 @@ class StateManager {
       const raw = localStorage.getItem(SAVE_KEY)
       if (!raw) return null
       const p = JSON.parse(raw) as GameStateData
-      if (p.version !== 4) return null
-      if (!p.world.crops)    p.world.crops    = {}
-      if (!p.world.villagers) p.world.villagers = {}
-      if (!p.world.stockpile) p.world.stockpile = {}
+      if (p.version !== 5) return null
+      if (!p.world.crops)          p.world.crops          = {}
+      if (!p.world.villagers)      p.world.villagers      = {}
+      if (!p.world.stockpile)      p.world.stockpile      = {}
+      if (!p.world.treeSeedlings)  p.world.treeSeedlings  = {}
+      if (!p.world.tileRecovery)   p.world.tileRecovery   = {}
       // Reset in-flight AI states to idle on load so runtime timers start fresh
       for (const v of Object.values(p.world.villagers)) {
         if (v.aiState === 'walking' || v.aiState === 'working') v.aiState = 'idle'