wip: checkpoint mid-refactor of asset loading before submesh work

Moves mesh/material loaders under src/assets/loaders, renames Mesh to
MeshHandle, and begins scaffolding the material loader. Known-broken
build state (MeshRegistry::from_scene call site and asset_manager
mesh registration are out of sync) — committed as a rollback point
before implementing submeshes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01986QNHM1cUsnRskKvBbbuo

src/assets/loaders/material/mod.rs

@@ -0,0 +1,2 @@
+pub mod mat_loader;
+pub mod gltf_mat_loader;