fix(materials): universal FailedMaterial sentinel for unmatched mesh objects

- export_gltf.py: replace single-material fallback (only fired when
  len(appended)==1) with a universal sentinel that appends
  SCHAEFFLER_059999_FailedMaterial unconditionally and assigns it to
  every mesh object not matched by name-based lookup.
  Also adds in-memory magenta fallback if library append fails.
  Removes 2 temporary [DEBUG] print lines from investigation.

- blender_render.py: add FailedMaterial assignment inside
  _apply_material_library() for unmatched parts (was log-only before).
  Includes copy-on-write guard (users > 1) matching existing pattern.

Also added alias 'Stahl; Durotect CMT' (semicolon) → Durotect-Blue
to cover STEP files using semicolon separator instead of comma.

Verified: 23/25 objects matched correctly, 2 ISO8734 dowel pins
(empty material) receive SCHAEFFLER_059999_FailedMaterial as sentinel.

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

render-worker/scripts/blender_render.py

@@ -482,7 +482,13 @@ def _apply_material_library(parts, mat_lib_path, mat_map):
 
     print(f"[blender_render] material assignment: {assigned_count}/{len(parts)} parts matched", flush=True)
     if unmatched_names:
-        print(f"[blender_render] unmatched parts (palette fallback): {unmatched_names[:10]}", flush=True)
+        print(f"[blender_render] unmatched parts → assigning {FAILED_MATERIAL_NAME}: {unmatched_names[:10]}", flush=True)
+        unmatched_set = set(unmatched_names)
+        for part in parts:
+            if part.name in unmatched_set:
+                if part.data.users > 1:
+                    part.data = part.data.copy()
+                _assign_failed_material(part)
 
 
 # ── Early GPU activation (must happen BEFORE open_mainfile / Cycles init) ────