✨ 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/TreeSeedlingSystem.ts

@@ -0,0 +1,131 @@
+import Phaser from 'phaser'
+import { TILE_SIZE, TREE_SEEDLING_STAGE_MS } from '../config'
+import { TileType, PLANTABLE_TILES } from '../types'
+import type { TreeSeedlingState } from '../types'
+import { stateManager } from '../StateManager'
+import type { LocalAdapter } from '../NetworkAdapter'
+import type { WorldSystem } from './WorldSystem'
+
+export class TreeSeedlingSystem {
+  private scene: Phaser.Scene
+  private adapter: LocalAdapter
+  private worldSystem: WorldSystem
+  private sprites = new Map<string, Phaser.GameObjects.Image>()
+
+  /**
+   * @param scene       - The Phaser scene this system belongs to
+   * @param adapter     - Network adapter for dispatching state actions
+   * @param worldSystem - Used to refresh the terrain canvas when a seedling matures
+   */
+  constructor(scene: Phaser.Scene, adapter: LocalAdapter, worldSystem: WorldSystem) {
+    this.scene = scene
+    this.adapter = adapter
+    this.worldSystem = worldSystem
+  }
+
+  /** Spawns sprites for all seedlings that exist in the saved state. */
+  create(): void {
+    const state = stateManager.getState()
+    for (const s of Object.values(state.world.treeSeedlings)) {
+      this.spawnSprite(s)
+    }
+  }
+
+  /**
+   * Ticks all seedling growth timers and handles stage changes.
+   * Stage 0→1: updates the sprite to the sapling texture.
+   * Stage 1→2: removes the seedling, spawns a tree resource, and updates the terrain canvas.
+   * @param delta - Frame delta in milliseconds
+   */
+  update(delta: number): void {
+    const advanced = stateManager.tickSeedlings(delta)
+    for (const id of advanced) {
+      const state = stateManager.getState()
+      const seedling = state.world.treeSeedlings[id]
+      if (!seedling) continue
+
+      if (seedling.stage === 2) {
+        // Fully mature: become a FOREST tile and a real tree resource
+        const { tileX, tileY } = seedling
+        this.removeSprite(id)
+        this.adapter.send({ type: 'REMOVE_TREE_SEEDLING', seedlingId: id })
+        this.adapter.send({ type: 'CHANGE_TILE', tileX, tileY, tile: TileType.FOREST })
+        this.worldSystem.refreshTerrainTile(tileX, tileY, TileType.FOREST)
+
+        const resourceId = `tree_grown_${tileX}_${tileY}_${Date.now()}`
+        this.adapter.send({
+          type: 'SPAWN_RESOURCE',
+          resource: { id: resourceId, tileX, tileY, kind: 'tree', hp: 3 },
+        })
+      } else {
+        // Stage 0→1: update sprite to sapling
+        const sprite = this.sprites.get(id)
+        if (sprite) sprite.setTexture(`seedling_${seedling.stage}`)
+      }
+    }
+  }
+
+  /**
+   * Attempts to plant a tree seedling on a grass tile.
+   * Validates that the stockpile has at least one tree_seed, the tile type is
+   * plantable (GRASS or DARK_GRASS), and no other object occupies the tile.
+   * @param tileX          - Target tile column
+   * @param tileY          - Target tile row
+   * @param underlyingTile - The current tile type (stored on the seedling for later restoration)
+   * @returns true if the seedling was planted, false if validation failed
+   */
+  plantSeedling(tileX: number, tileY: number, underlyingTile: TileType): boolean {
+    const state = stateManager.getState()
+
+    if ((state.world.stockpile.tree_seed ?? 0) <= 0) return false
+    if (!PLANTABLE_TILES.has(underlyingTile)) return false
+
+    const occupied =
+      Object.values(state.world.resources).some(r => r.tileX === tileX && r.tileY === tileY) ||
+      Object.values(state.world.buildings).some(b => b.tileX === tileX && b.tileY === tileY) ||
+      Object.values(state.world.crops).some(c => c.tileX === tileX && c.tileY === tileY) ||
+      Object.values(state.world.treeSeedlings).some(s => s.tileX === tileX && s.tileY === tileY)
+
+    if (occupied) return false
+
+    const id = `seedling_${tileX}_${tileY}_${Date.now()}`
+    const seedling: TreeSeedlingState = {
+      id, tileX, tileY,
+      stage: 0,
+      stageTimerMs: TREE_SEEDLING_STAGE_MS,
+      underlyingTile,
+    }
+
+    this.adapter.send({ type: 'PLANT_TREE_SEED', seedling })
+    this.spawnSprite(seedling)
+    return true
+  }
+
+  /**
+   * Creates and registers the sprite for a seedling.
+   * @param s - Seedling state to render
+   */
+  private spawnSprite(s: TreeSeedlingState): void {
+    const x = (s.tileX + 0.5) * TILE_SIZE
+    const y = (s.tileY + 0.5) * TILE_SIZE
+    const key = `seedling_${Math.min(s.stage, 2)}`
+    const sprite = this.scene.add.image(x, y, key)
+      .setOrigin(0.5, 0.85)
+      .setDepth(5)
+    this.sprites.set(s.id, sprite)
+  }
+
+  /**
+   * Destroys the sprite for a seedling and removes it from the registry.
+   * @param id - Seedling ID
+   */
+  private removeSprite(id: string): void {
+    const s = this.sprites.get(id)
+    if (s) { s.destroy(); this.sprites.delete(id) }
+  }
+
+  /** Destroys all seedling sprites and clears the registry. */
+  destroy(): void {
+    for (const id of [...this.sprites.keys()]) this.removeSprite(id)
+  }
+}