feat(M5-M7): embed canonical material names in USD via customData + pxr direct read

- export_step_to_usd.py: accept --material_map CLI arg, write
  schaeffler:canonicalMaterialName as customData on each Mesh prim,
  fix geometry transform (strip shape Location before face exploration,
  apply both face_loc and shape_loc sequentially)
- import_usd.py: after Blender USD import, use pxr to read customData
  directly from the USD file — builds {part_key: material_name} lookup
  (Blender ignores STRING primvars and customData, but pxr reads both)
- _blender_materials.py: add apply_material_library_direct() for exact
  dict-based material assignment without name-matching heuristics
- _blender_scene_setup.py: prefer direct USD lookup, fall back to
  name-matching for legacy USD files without material metadata
- export_glb.py (generate_usd_master_task): resolve material_map via
  material_service.resolve_material_map() and pass to subprocess;
  include material hash in cache key for invalidation
- ROADMAP.md: update P5 status, add M5-M7 milestones

Tested: 3/3 parts matched (ans_lfs120), 172/175 parts matched
(F-802007.TR4-D1-H122AG). Previous: 0/25 matched.

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

render-worker/scripts/import_usd.py

@@ -15,10 +15,14 @@ import bmesh    # type: ignore[import]
 from mathutils import Vector  # type: ignore[import]
 
 
-def import_usd_file(usd_path: str) -> list:
+def import_usd_file(usd_path: str) -> list | tuple:
     """Import USD stage into current Blender scene.
 
-    Returns list of imported mesh objects, centred at world origin.
+    Returns a tuple of (parts, material_lookup) where:
+    - parts: list of imported mesh objects, centred at world origin
+    - material_lookup: dict mapping blender_object_name → canonical_material_name
+      (populated from schaeffler:canonicalMaterialName primvars, empty dict if absent)
+
     USD stage is mm Y-up with metersPerUnit=0.001 — Blender scales to metres.
     """
     bpy.ops.object.select_all(action='DESELECT')
@@ -43,6 +47,38 @@ def import_usd_file(usd_path: str) -> list:
     if restored:
         print(f"[import_usd] restored seam/sharp on {restored} mesh(es)", flush=True)
 
+    # Extract material lookup via pxr direct read of the USD file.
+    # Blender's USD importer does NOT expose STRING primvars or customData as
+    # Python-accessible properties — but the pxr module (available in render-worker)
+    # can read them perfectly from the same file.
+    material_lookup: dict[str, str] = {}
+    try:
+        from pxr import Usd, UsdGeom  # type: ignore[import]
+        stage = Usd.Stage.Open(usd_path)
+        for prim in stage.Traverse():
+            if prim.GetTypeName() != "Mesh":
+                continue
+            part_key = prim.GetCustomDataByKey("schaeffler:partKey") or ""
+            mat_name = prim.GetCustomDataByKey("schaeffler:canonicalMaterialName") or ""
+            if not part_key or not mat_name:
+                # Also check parent Xform prim (metadata may be on container)
+                parent = prim.GetParent()
+                if parent:
+                    part_key = part_key or (parent.GetCustomDataByKey("schaeffler:partKey") or "")
+                    mat_name = mat_name or (parent.GetCustomDataByKey("schaeffler:canonicalMaterialName") or "")
+            if part_key and mat_name:
+                # Blender object name = mesh prim leaf name (part_key)
+                material_lookup[part_key] = mat_name
+    except Exception as exc:
+        print(f"[import_usd] WARNING: pxr material lookup failed: {exc}", flush=True)
+
+    if material_lookup:
+        print(f"[import_usd] pxr material lookup: {len(material_lookup)}/{len(parts)} parts",
+              flush=True)
+    else:
+        print("[import_usd] no schaeffler:canonicalMaterialName metadata found (legacy USD)",
+              flush=True)
+
     # Centre combined bbox at world origin (same as import_glb convention)
     all_corners = []
     for p in parts:
@@ -60,7 +96,7 @@ def import_usd_file(usd_path: str) -> list:
         for obj in root_objects:
             obj.location -= center
 
-    return parts
+    return parts, material_lookup
 
 
 def _rename_usd_objects(parts: list) -> None: