- Introduce HitboxID, EntityID, and ModelID types for safer ID-based lookups
- Replace AABB struct with Hitbox (includes HitboxID)
- Convert ModelManager and HitboxManager to singletons with Handle structs
- Update Entity to store IDs for model and hitbox, removing direct references
- Reorganize creature model assets into subdirectories per entity type
- Add player model (player.glb) and collision data for pig
- Remove ModelManager dependency from App and Renderer
- Add namespace parsing utility for asset paths
- Mark sparse_vector::insert() with [[nodiscard]]
