Mushrooms, cave level, separate physicsTick from tick
- Add mushrooms.blend with 31 mushroom mesh variants
- Add cave.blend with CrystalSanctuary mesh and dedicated collision
- Place 31 mushroom instances in gameplay.json near starting area
- Update CrystalSanctuary path from world/ to cave/ with separate coll
- Filter scene editor asset list to mushrooms only
- Extract Scene::physicsTick from Scene::tick for clarity