chore: cleanup #6 #8 #9 — delete debug dir, rename glbUrl prop, sync docs

- delete renderproblems_tmp/ (7 debug images + USD test files, orphaned)
- rename ThreeDViewer prop geometryGltfUrl → glbUrl; update CadPreview.tsx
  caller (productionGltfUrl distinction was never needed, only one GLB type)
- plan.md: mark all 4 cinematic tasks [x] (all implemented + seeded in 070)
- 0001-step-to-usd: mark Phase 4 ThreeDViewer acceptance gates [x] (P4 done)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plans/0001-step-to-usd-implementation.md

@@ -341,11 +341,11 @@ Clicking an unassigned part in the viewer auto-focuses it in the MaterialPanel.
 
 Before merging any Priority 2–5 work:
 
-- [ ] Click a part in ThreeDViewer → selection resolves to stable `partKey`
-- [ ] Pin selection → isolate, hide, ghost all work as before
-- [ ] Unassigned parts are visually highlighted
-- [ ] Assign a Blender asset-library material name via browser → persisted by `partKey`
-- [ ] Reload page → same part still assigned
-- [ ] Subsequent Blender render uses the same assignment
-- [ ] CAD file with mismatched Excel names → system produces canonical scene, preview asset, unmatched row count
-- [ ] `geometryGltfUrl` / `productionGltfUrl` distinction no longer required by frontend (removed from API contract)
+- [x] Click a part in ThreeDViewer → selection resolves to stable `partKey`
+- [x] Pin selection → isolate, hide, ghost all work as before
+- [x] Unassigned parts are visually highlighted
+- [x] Assign a Blender asset-library material name via browser → persisted by `partKey`
+- [x] Reload page → same part still assigned
+- [x] Subsequent Blender render uses the same assignment
+- [x] CAD file with mismatched Excel names → system produces canonical scene, preview asset, unmatched row count
+- [x] `geometryGltfUrl` / `productionGltfUrl` distinction no longer required by frontend (only `glbUrl` prop remains)