fix(export_gltf): clear OCC custom_normal attribute before sharp edge processing

The geometry GLB from export_step_to_gltf.py contains a 'custom_normal' attribute
(CORNER, INT16_2D) from OCC tessellation. If left in place, Blender's glTF exporter
re-exports these pre-baked normals unchanged — ignoring shade_smooth_by_angle
processing and our explicit sharp edge marks.

Fix: remove the 'custom_normal' attribute from all imported mesh objects immediately
after GLB import, before applying smooth shading. Also add orphans_purge() before
export to remove palette materials (mat_0/1/2/3) that become users=0 after library
material substitution.

Same custom_normal clearing applied to blender_render.py for thumbnail renders.

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

render-worker/scripts/blender_render.py

@@ -342,6 +342,16 @@ def _import_glb(glb_file):
     print(f"[blender_render] imported {len(parts)} part(s) from GLB: "
           f"{[p.name for p in parts[:5]]}")
 
+    # Remove OCC-baked custom normals so shade_smooth_by_angle can recompute
+    # normals from scratch (respecting our sharp edge marks).
+    cleared = 0
+    for p in parts:
+        if "custom_normal" in p.data.attributes:
+            p.data.attributes.remove(p.data.attributes["custom_normal"])
+            cleared += 1
+    if cleared:
+        print(f"[blender_render] cleared OCC custom_normal from {cleared} mesh objects")
+
     # Centre combined bbox at world origin
     all_corners = []
     for p in parts:

render-worker/scripts/export_gltf.py

@@ -116,6 +116,20 @@ def main() -> None:
     mesh_objects = [o for o in bpy.data.objects if o.type == "MESH"]
     print(f"Imported geometry GLB: {args.glb_path} ({len(mesh_objects)} mesh objects)")
 
+    # Remove OCC-baked custom normals from the geometry GLB.
+    # RWGltf_CafWriter embeds per-corner normals from OCC tessellation as a
+    # 'custom_normal' attribute (CORNER, INT16_2D). If left in place, Blender's
+    # glTF exporter re-exports these pre-baked normals unchanged, ignoring our
+    # shade_smooth_by_angle processing and sharp edge marks entirely.
+    # Removing this attribute forces Blender to recompute normals from scratch.
+    cleared_normals = 0
+    for obj in mesh_objects:
+        if "custom_normal" in obj.data.attributes:
+            obj.data.attributes.remove(obj.data.attributes["custom_normal"])
+            cleared_normals += 1
+    if cleared_normals:
+        print(f"Cleared OCC custom_normal attribute from {cleared_normals} mesh objects")
+
     # Apply smooth shading using the configured angle threshold
     smooth_rad = _math.radians(args.smooth_angle)
     print(f"Applying smooth shading at {args.smooth_angle}° ({smooth_rad:.3f} rad)")
@@ -235,6 +249,14 @@ def main() -> None:
                     if fallback:
                         print(f"Single-material fallback: applied '{default_mat_name}' to {fallback} unmatched objects")
 
+    # Purge orphan data-blocks (palette materials mat_0/mat_1/... from the geometry
+    # GLB that now have users=0 after library material substitution).
+    # This prevents stale materials from appearing as duplicates in the export.
+    try:
+        bpy.ops.outliner.orphans_purge(do_recursive=True)
+    except Exception:
+        pass  # non-critical; export proceeds regardless
+
     # Export production GLB with full PBR material data
     try:
         bpy.ops.export_scene.gltf(