✨ add mine building for automated stone production

- New 3×2 building placeable only on ROCK tiles without resources (50 stone + 200 wood)
- Mine entrance at bottom-centre tile (tileX+1, tileY+1) — only passable tile
- Other 5 footprint tiles blocked via resourceTiles index (impassable ROCK)
- Nisse with mine priority > 0 walk to entrance, hide inside for MINE_WORK_MS (15 s),
  then reappear carrying MINE_STONE_YIELD (2) stone and haul to stockpile
- Up to MINE_CAPACITY (3) Nisse work simultaneously; overflow Nisse wait for a slot
- ⛏ X/3 world-space status label above building updated each frame
- Surface rock harvesting unchanged; mine buildings take precedence in pickJob
- Ghost resizes to 3×2 when mine is selected; placement validated across full footprint
- Mine added to build menu with cost and placement hint

Closes #42

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scenes/UIScene.ts

@@ -229,9 +229,10 @@ export class UIScene extends Phaser.Scene {
       { kind: 'bed',            label: '🛏 Bed',          cost: '6 wood  (+1 villager)' },
       { kind: 'stockpile_zone', label: '📦 Stockpile',   cost: 'free  (workers deliver here)' },
       { kind: 'forester_hut',   label: '🌲 Forester Hut', cost: '50 wood' },
+      { kind: 'mine',           label: '⛏ Mine',          cost: '200 wood + 50 stone  (place on ROCK)' },
     ]
-    const menuX = this.scale.width / 2 - 150, menuY = this.scale.height / 2 - 168
-    const bg = this.add.rectangle(menuX, menuY, 300, 326, 0x000000, this.uiOpacity).setOrigin(0,0).setScrollFactor(0).setDepth(200)
+    const menuX = this.scale.width / 2 - 150, menuY = this.scale.height / 2 - 186
+    const bg = this.add.rectangle(menuX, menuY, 300, 372, 0x000000, this.uiOpacity).setOrigin(0,0).setScrollFactor(0).setDepth(200)
     this.buildMenuGroup.add(bg)
     this.buildMenuGroup.add(this.add.text(menuX + 150, menuY + 14, 'BUILD MENU  [B/ESC]', { fontSize: '11px', color: '#aaaaaa', fontFamily: 'monospace' }).setOrigin(0.5,0).setScrollFactor(0).setDepth(201))