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

test_scene.scene

@@ -1,13 +1,12 @@
 {
     "name": "Testing Scene",
     "mesh_paths": {
-        "gunship": "meshes/laat_gunship/scene.obj",
         "trooper": "meshes/clone_trooper/scene.gltf",
-        "destroyer": "../meshes/sw_isd.glb",
-        "interceptor": "../meshes/sw_tie_interceptor.glb"
+        "gunship": "meshes/laat_gltf/scene.gltf",
+        "helmet": "meshes/DamagedHelmet.glb"
     },
     "instances": {
-        "interceptor": {
+        "gunship": {
             "translation": [
                 0.0,
                 0.0,
@@ -15,7 +14,7 @@
             ],
             "rotation": [
                 0.0,
-                0.0,
+                3.14,
                 0.0
             ],
             "scale": [