🐛 keep ROCK tile type after surface rock mining

CHANGE_TILE ROCK→GRASS removed from the surface-rock harvest branch.
Empty ROCK tiles are now passable (no resource on them) but remain ROCK,
so the mine building can still be placed on harvested rock ground.

Fixes #48

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

src/systems/VillagerSystem.ts

@@ -296,7 +296,8 @@ export class VillagerSystem {
       const res = state.world.resources[job.targetId]
       if (res) {
         this.adapter.send({ type: 'VILLAGER_HARVEST_RESOURCE', villagerId: v.id, resourceId: job.targetId })
-        this.adapter.send({ type: 'CHANGE_TILE', tileX: res.tileX, tileY: res.tileY, tile: TileType.GRASS })
+        // ROCK tile stays ROCK after mining — empty rocky ground remains passable
+        // and valid for mine building placement.
         this.worldSystem.removeResourceTile(res.tileX, res.tileY)
         this.resourceSystem.removeResource(job.targetId)
         this.addLog(v.id, '✓ Mined rock (+2 stone)')