feat: performance optimizations + part-materials validation

- @timed_step decorator with wall-clock + RSS tracking (pipeline_logger)
- Blender timing laps for sharp edges and material assignment
- MeshRegistry pattern: eliminate 13 scene.traverse() calls across viewers
- Lazy material cloning (clone-on-first-write in both viewers)
- _pipeline_session context manager: 7 create_engine() → 2 in render_thumbnail
- KD-tree spatial pre-filter for sharp edge marking (bbox-based pruning)
- Batch material library append: N bpy.ops.wm.append → single bpy.data.libraries.load
- GMSH single-session batching: compound all solids into one tessellation call
- Validate part-materials save endpoints against parsed_objects (prevents bogus keys)
- ROADMAP updated with completion status

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

frontend/src/components/cad/cadUtils.ts

@@ -172,3 +172,27 @@ export function previewColorForEntry(
   }
   return '#888888'
 }
+
+// ---------------------------------------------------------------------------
+// MeshRegistry — O(1) access to meshes by partKey, replaces O(n) traversals
+// ---------------------------------------------------------------------------
+
+/** A single entry in the mesh registry, linking a Three.js mesh to its partKey. */
+export interface MeshRegistryEntry {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  mesh: any  // THREE.Mesh — typed as any to avoid importing THREE
+  partKey: string
+}
+
+/**
+ * Iterate all materials on a mesh, calling `fn` for each MeshStandardMaterial.
+ * Handles both single and array materials safely.
+ */
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function forEachMeshMaterial(mesh: any, fn: (mat: any) => void): void {
+  if (!mesh?.material) return
+  const mats = Array.isArray(mesh.material) ? mesh.material : [mesh.material]
+  for (const m of mats) {
+    if (m && 'color' in m) fn(m)
+  }
+}