feat: surface-evaluated normals, GMSH tessellation, draw call batching

USD exporter:
- Compute normals from B-Rep surface via BRepLProp_SLProps at each vertex
  UV parameter — eliminates faceting on curved surfaces (same as Stepper)
- Add GMSH Frontal-Delaunay tessellation engine (opt-in via --tessellation_engine gmsh)
  with per-solid strategy matching export_step_to_gltf.py
- Use vertex normal interpolation instead of faceVarying (6x smaller normals)
- Default engine remains OCC (GMSH has coordinate-space bug with instanced parts)

Frontend:
- Fix faceted shading in InlineCadViewer: only call computeVertexNormals()
  when geometry lacks normals, preserving smooth GLB normals from pipeline
- Add useGeometryMerge hook for draw call batching (merge by material)
- Fix unused import in cadUtils, optional props in ThreeDViewer

Backend:
- Move dataclass import to top of step_processor.py (PEP 8)
- Unified single-read STEP metadata extraction with fallback

Render worker:
- Fix USD import seam/sharp restoration: read primvars via pxr directly
  (Blender's USD importer doesn't expose custom Int2Array primvars)

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

frontend/src/components/cad/InlineCadViewer.tsx

@@ -4,7 +4,7 @@ import { Canvas, useThree } from '@react-three/fiber'
 import { OrbitControls, useGLTF, Environment } from '@react-three/drei'
 import * as THREE from 'three'
 import { mergeVertices } from 'three/examples/jsm/utils/BufferGeometryUtils.js'
-import { Loader2, Box, RefreshCw, Grid3X3, Layers, Sun, AlertCircle, EyeOff } from 'lucide-react'
+import { Loader2, Box, RefreshCw, Grid3X3, Layers, Sun, AlertCircle, EyeOff, Zap } from 'lucide-react'
 import { toast } from 'sonner'
 import { listMediaAssets as getMediaAssets } from '../../api/media'
 import { generateGltfGeometry, getPartMaterials, type PartMaterialMap } from '../../api/cad'
@@ -12,6 +12,7 @@ import { useAuthStore } from '../../store/auth'
 import MaterialPanel, { type IsolateMode } from './MaterialPanel'
 import { normalizeMeshName, resolvePartMaterial, remapToPartKeys, applyPBRToMaterial, previewColorForEntry, forEachMeshMaterial, type MeshRegistryEntry } from './cadUtils'
 import { fetchMaterialPBR, type MaterialPBRMap } from '../../api/assetLibraries'
+import { useGeometryMerge } from './useGeometryMerge'
 
 type ViewMode = 'solid' | 'wireframe'
 type LightPreset = 'studio' | 'warehouse' | 'sunset' | 'park' | 'city'
@@ -105,7 +106,10 @@ function GlbModelWithFit({
         if (obj.geometry) {
           let geo = obj.geometry.clone()
           if (!geo.index) geo = mergeVertices(geo)
-          geo.computeVertexNormals()
+          // Only compute normals if the geometry doesn't already have them.
+          // GLBs from our pipeline include smooth normals — overwriting them
+          // with computeVertexNormals() produces flat/faceted shading.
+          if (!geo.attributes.normal) geo.computeVertexNormals()
           obj.geometry = geo
         }
         // Clone materials so emissive / color changes don't affect the shared GLTF cache
@@ -189,6 +193,7 @@ export default function InlineCadViewer({
   const [showUnassigned, setShowUnassigned] = useState(false)
   const [hideAssigned, setHideAssigned] = useState(false)
   const [isolateMode, setIsolateMode] = useState<IsolateMode>('none')
+  const [perfMode, setPerfMode] = useState(false)
   const [totalMeshCount, setTotalMeshCount] = useState(0)
   const [glbMeshNames, setGlbMeshNames] = useState<Set<string>>(new Set())
   const [partKeyMap, setPartKeyMap] = useState<Record<string, string>>({})
@@ -381,6 +386,15 @@ export default function InlineCadViewer({
     onError: () => toast.error('Failed to queue GLB generation'),
   })
 
+  // Performance mode: merge geometries by material to reduce draw calls
+  useGeometryMerge({
+    meshRegistryRef,
+    partMaterials,
+    pbrMap,
+    enabled: perfMode,
+    sceneRef,
+  })
+
   // Hover highlight
   const handlePointerOver = useCallback((e: any) => {
     e.stopPropagation()
@@ -469,6 +483,13 @@ export default function InlineCadViewer({
             </ToolbarBtn>
           ))}
 
+          <div className="w-px h-4 bg-white/10 mx-0.5" />
+
+          {/* Performance mode */}
+          <ToolbarBtn active={perfMode} onClick={() => setPerfMode(v => !v)} title="Performance mode — merges geometries, disables per-part hover">
+            <Zap size={11} /> Perf
+          </ToolbarBtn>
+
           {/* Show unassigned + hide assigned toggles */}
           {modelReady && (
             <>
@@ -542,9 +563,9 @@ export default function InlineCadViewer({
                   setModelReady(true)
                   setFitTrigger(t => t + 1)
                 }}
-                onPointerOver={handlePointerOver}
-                onPointerOut={handlePointerOut}
-                onClick={handleClick}
+                onPointerOver={perfMode ? undefined : handlePointerOver}
+                onPointerOut={perfMode ? undefined : handlePointerOut}
+                onClick={perfMode ? undefined : handleClick}
               />
             </Suspense>
             <OrbitControls ref={controlsRef} makeDefault />