refactor(gameplay): integrate model and hitbox ID system into Entity

- 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]]

assets/model/creature/pig/collision.json

@@ -0,0 +1,16 @@
+{
+    "boxes": [
+        {
+            "center": [
+                0,
+                0.78,
+                0
+            ],
+            "half": [
+                0.53,
+                0.78,
+                0.406
+            ]
+        }
+    ]
+}