✨ 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/config.ts

@@ -20,8 +20,18 @@ export const BUILDING_COSTS: Record<BuildingType, Record<string, number>> = {
   bed:           { wood: 6 },
   stockpile_zone:{ wood: 0 },
   forester_hut:  { wood: 50 },
+  mine:          { stone: 50, wood: 200 },
 }
 
+/** Maximum number of Nisse that can work inside a mine simultaneously. */
+export const MINE_CAPACITY = 3
+
+/** Milliseconds a Nisse spends inside a mine per visit. */
+export const MINE_WORK_MS = 15_000
+
+/** Stone yielded per mine visit. */
+export const MINE_STONE_YIELD = 2
+
 /** Max Chebyshev radius (in tiles) that a forester hut's zone can extend. */
 export const FORESTER_ZONE_RADIUS = 5