- 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]]
- Extract `ModelID` and `Model` struct to new `model.hpp`
- Replace `std::unordered_map` with `tbb::concurrent_hash_map` for thread safety
- Add `get_model(ModelID)`, `get_model_id`, `get_model_name` methods
- Parse model names in `namespace:name` format to construct asset paths
- Update `load_model` to accept `string_view` and use ID-based management