Import: multi-mesh / multi-primitive models (submeshes + per-primitive materials)

The glTF/OBJ importer currently cooks only the first mesh's first primitive (engine/vendor/cgltf_wrap.c, engine/assets/GltfLoader.zig, and the canonical mesh cooker in editor/AssetImporter.zig). Real scenes — Sponza, Bistro, the Khronos ChronographWatch — have many meshes/primitives, each with its own material, so most geometry is silently dropped today.

Scope

• Cook all primitives of all meshes into a multi-submesh canonical mesh (extend the TMSH format with a submesh table: index range + material slot).
• Generate/assign a material per primitive (the one-to-many manifest already supports many materials).
• MeshRenderer (or a new submesh-aware path) renders each submesh with its material in both the render module and the software renderer.
• Editor: when instantiating a model, create the submesh material bindings.

Why

Blocks importing complex models/scenes (the stated Sponza/Bistro goal). Follows the import pipeline established in #16 (closed).