feat: OBJ model loader + fix mesh.h duplicates

- mesh_load_obj: parse vertices, UVs, faces from .obj files
- Auto-generate face normals
- Per-face vertex expansion (simple but correct)

Engine/Rendering/mesh.h

@@ -20,5 +20,6 @@ typedef struct {
 Mesh* mesh_create_cube(ID3D11Device* dev);
 Mesh* mesh_create_plane(ID3D11Device* dev, float size);
 Mesh* mesh_create_sphere(ID3D11Device* dev, float r, int seg);
+Mesh* mesh_load_obj(ID3D11Device* dev, const char* path);
 void mesh_destroy(Mesh* m);
 void mesh_draw(Mesh* m, ID3D11DeviceContext* ctx);