✨ 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>

CHANGELOG.md

@@ -7,6 +7,15 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Added
+- **Unified Tile System** (Issue #14):
+  - Tree seedlings: player plants `tree_seed` on grass/dark-grass via the F-key farming tool; seedling grows through two stages (sprout → sapling → young tree, ~1 min each); on maturity it becomes a FOREST tile with a harvestable tree resource
+  - Tile recovery: when a Nisse chops a tree, the resulting DARK_GRASS tile starts a 5-minute recovery timer and reverts to GRASS automatically, with the terrain canvas updated in real time
+  - Three new procedural seedling textures (`seedling_0/1/2`) generated in BootScene
+  - `tree_seed` added to stockpile display (default 5 at game start) and to the farming tool cycle
+  - `WorldSystem.refreshTerrainTile()` updates the terrain canvas for a single tile without regenerating the full background
+  - New `TreeSeedlingSystem` manages seedling sprites, growth ticking, and maturation
+
 ### Added
 - **Nisse Info Panel** (Issue #9): clicking a Nisse opens a top-left panel with name, AI status, energy bar, active job, job priority buttons, and a live work log (last 10 of 20 runtime entries); closes with ESC, ✕ button, or by clicking another Nisse
 - Work log tracks: walking to job, hauling to stockpile, going to sleep, waking up, chopped/mined/farmed results, deposited at stockpile