fix(USD): correct coordinate transform (Z-up stage) and store canonical_material in DB

- export_step_to_usd.py: change stage from Y-up to Z-up, keep (X,-Z,Y)
  transform — matches GLB orientation exactly (verified: bounding box match)
- export_glb.py: include canonical_material in resolved_material_assignments
  DB field (was being dropped during manifest parsing)
- import_usd.py: use pxr customData read (not string primvars — Blender
  ignores those, confirmed by testing)

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

render-worker/scripts/export_step_to_usd.py

@@ -634,7 +634,7 @@ def main() -> None:
 
     # ── Create USD stage ──────────────────────────────────────────────────────
     stage = Usd.Stage.CreateNew(str(output_path))
-    UsdGeom.SetStageUpAxis(stage, UsdGeom.Tokens.y)
+    UsdGeom.SetStageUpAxis(stage, UsdGeom.Tokens.z)
     UsdGeom.SetStageMetersPerUnit(stage, 0.001)  # mm; Blender handles m conversion on import
 
     root_prim = UsdGeom.Xform.Define(stage, "/Root")
@@ -705,7 +705,10 @@ def main() -> None:
             mesh = UsdGeom.Mesh.Define(stage, mesh_path)
             mesh.CreateSubdivisionSchemeAttr(UsdGeom.Tokens.none)
 
-            # OCC (X, Y, Z) mm Z-up → USD (X, -Z, Y) mm Y-up
+            # OCC is Z-up (mm) but Y-forward. Blender is Z-up, Y-backward.
+            # GLB export uses: Blender(X, -Z_occ, Y_occ) × 0.001
+            # USD stage is Z-up with metersPerUnit=0.001, so Blender applies
+            # only the scale.  Write (X, -Z, Y) to match GLB orientation.
             mesh.CreatePointsAttr(Vt.Vec3fArray([
                 Gf.Vec3f(x, -z, y) for (x, y, z) in vertices
             ]))