fix(export_gltf): use edit-mode mark_sharp+mark_seam for proper GLB sharp edges

Replace shade_smooth_by_angle with explicit edit-mode operators:
- edges_select_sharp(angle) → mark_sharp() + mark_seam()
- Produces vertex splits at sharp edges (6027 split positions verified)
- Remove OCC custom_normal attribute before processing to prevent
  pre-baked normals overriding Blender's sharp edge processing
- Update comment: calc_normals_split() removed in Blender 5.0

Verified: production GLB has 812 extra vertices vs geometry GLB,
6027 positions with multiple normals = sharp edges correctly encoded.

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

render-worker/scripts/export_gltf.py

@@ -130,21 +130,45 @@ def main() -> None:
     if cleared_normals:
         print(f"Cleared OCC custom_normal attribute from {cleared_normals} mesh objects")
 
-    # Apply smooth shading using the configured angle threshold
+    # Mark sharp edges and seams using the configured angle threshold.
+    # We use Blender's edit-mode operators (mark_sharp + mark_seam) rather than
+    # shade_smooth_by_angle alone, because:
+    # 1. mark_sharp() sets the sharp_edge boolean attribute on edges — the glTF
+    #    exporter creates vertex splits (duplicate vertices with different normals)
+    #    at sharp edges, which is how glTF encodes hard edges.
+    # 2. mark_seam() ensures UV splits at the same edges (stepper-addon behaviour).
+    # Note: calc_normals_split() was removed in Blender 5.0 — not needed here
+    #       because export_apply=True triggers vertex splitting automatically.
     smooth_rad = _math.radians(args.smooth_angle)
-    print(f"Applying smooth shading at {args.smooth_angle}° ({smooth_rad:.3f} rad)")
+    print(f"Marking sharp edges + seams at {args.smooth_angle}° ({smooth_rad:.3f} rad)")
+
+    bpy.ops.object.select_all(action='DESELECT')
+    total_sharp = 0
     for obj in mesh_objects:
         bpy.context.view_layer.objects.active = obj
         obj.select_set(True)
-        try:
-            bpy.ops.object.shade_smooth_by_angle(angle=smooth_rad)
-        except Exception:
-            bpy.ops.object.shade_smooth()
-            if hasattr(obj.data, 'use_auto_smooth'):
-                obj.data.use_auto_smooth = True
-                obj.data.auto_smooth_angle = smooth_rad
 
-    # Apply OCC sharp edges if available (overrides pure dihedral-angle shading)
+        # Set all faces smooth
+        bpy.ops.object.mode_set(mode='OBJECT')
+        for poly in obj.data.polygons:
+            poly.use_smooth = True
+
+        # Enter edit mode, deselect, select sharp edges by angle, mark sharp+seam
+        bpy.ops.object.mode_set(mode='EDIT')
+        bpy.ops.mesh.select_all(action='DESELECT')
+        bpy.ops.mesh.edges_select_sharp(sharpness=smooth_rad)
+        bpy.ops.mesh.mark_sharp()
+        bpy.ops.mesh.mark_seam()
+        bpy.ops.object.mode_set(mode='OBJECT')
+
+        # Count how many edges were marked
+        n_sharp = sum(1 for e in obj.data.edges if e.use_edge_sharp)
+        total_sharp += n_sharp
+        obj.select_set(False)
+
+    print(f"Marked {total_sharp} sharp/seam edges across {len(mesh_objects)} objects")
+
+    # Apply OCC sharp edges if available (additional explicit sharp edges from CAD data)
     sharp_pairs = mesh_attributes.get("sharp_edge_pairs") or []
     if sharp_pairs:
         _apply_sharp_edges_from_occ(mesh_objects, sharp_pairs)