✨ 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/scenes/BootScene.ts

@@ -20,6 +20,7 @@ export class BootScene extends Phaser.Scene {
     this.buildResourceTextures()
     this.buildPlayerTexture()
     this.buildCropTextures()
+    this.buildSeedlingTextures()
     this.buildUITextures()
     this.buildVillagerAndBuildingTextures()
     this.generateWorldIfNeeded()
@@ -287,6 +288,40 @@ export class BootScene extends Phaser.Scene {
     g3.generateTexture('crop_carrot_3', W, H); g3.destroy()
   }
 
+  // ─── Tree seedling textures (3 growth stages) ────────────────────────────
+
+  /**
+   * Generates textures for the three tree-seedling growth stages:
+   *   seedling_0 – small sprout
+   *   seedling_1 – sapling with leaves
+   *   seedling_2 – young tree (about to mature into a FOREST tile)
+   */
+  private buildSeedlingTextures(): void {
+    // Stage 0: tiny sprout
+    const g0 = this.add.graphics()
+    g0.fillStyle(0x6D4C41); g0.fillRect(10, 20, 4, 10)
+    g0.fillStyle(0x66BB6A); g0.fillEllipse(12, 16, 12, 8)
+    g0.fillStyle(0x4CAF50); g0.fillEllipse(12, 13, 8, 6)
+    g0.generateTexture('seedling_0', 24, 32); g0.destroy()
+
+    // Stage 1: sapling
+    const g1 = this.add.graphics()
+    g1.fillStyle(0x6D4C41); g1.fillRect(9, 15, 5, 16)
+    g1.fillStyle(0x4CAF50); g1.fillCircle(12, 12, 8)
+    g1.fillStyle(0x66BB6A, 0.7); g1.fillCircle(7, 16, 5); g1.fillCircle(17, 16, 5)
+    g1.fillStyle(0x81C784); g1.fillCircle(12, 8, 5)
+    g1.generateTexture('seedling_1', 24, 32); g1.destroy()
+
+    // Stage 2: young tree (mature, ready to become a resource)
+    const g2 = this.add.graphics()
+    g2.fillStyle(0x000000, 0.15); g2.fillEllipse(12, 28, 16, 6)
+    g2.fillStyle(0x6D4C41); g2.fillRect(9, 14, 6, 14)
+    g2.fillStyle(0x2E7D32); g2.fillCircle(12, 9, 10)
+    g2.fillStyle(0x388E3C); g2.fillCircle(7, 13, 7); g2.fillCircle(17, 13, 7)
+    g2.fillStyle(0x43A047); g2.fillCircle(12, 6, 7)
+    g2.generateTexture('seedling_2', 24, 32); g2.destroy()
+  }
+
   // ─── UI panel texture ─────────────────────────────────────────────────────
 
   private buildUITextures(): void {