Import glTF/GLB meshes & scene hierarchy (materials → #31)

Current State

engine/assets/GltfLoader.zig + engine/vendor/cgltf_wrap.c extract only basic mesh geometry from a single primitive. Node hierarchy, multi-primitive meshes, naming, and tangents are dropped. There is also a separate ObjLoader.zig.

Scope split: This issue covers mesh + scene-graph import. glTF material import is tracked separately in #16 (closed), and the material system itself in. Re-scoped from XL → L accordingly.

Goal

Robust import of glTF/GLB geometry and node structure, with other formats (OBJ) converted through the same internal mesh representation at import time.

Tasks

• Extend the cgltf wrapper to walk all meshes/primitives (not just primitive 0)
• Import node hierarchy → GameObject tree with local transforms and names
• Import normals; generate UVs/tangents when missing (tangents needed for normal mapping)
• Handle indexed and non-indexed primitives, and multiple UV sets
• Preserve mesh/node names for stable identity
• Route OBJ import through the same internal mesh path (dedupe ObjLoader)
• Generate .meta + stable GUIDs for imported meshes; reimport is idempotent
• Defer materials/textures to #16 (closed) (assign placeholder material for now)

Acceptance Criteria

• Importing a multi-mesh.gltf/.glb produces the correct GameObject hierarchy with named nodes and transforms.
• Meshes render in the viewport and the built game.
• Reimport yields stable GUIDs (no duplicate assets).

Related

• #16 (closed) (glTF→materials), (material system), #5 (closed) (texture importer).