feat(paint-slots): paintable slots for windows + doors (frame/glass, panel/glass)

Windows and doors build all visuals in their viewer systems from module-global
materials, so this threads per-node slot materials + userData.slotId tags
through those builders without restructuring them:

- window: 'frame' + 'glass' slots. door: 'panel' (body = casing + leaf) +
  'glass'; the opening reveal keeps its own material.
- Each system captures per-frame viewer state, then updateWindow/DoorMesh points
  the builder-facing base/glass materials at the node's resolved slot override
  (recomputed per node, so the next node resets without a restore). Meshes are
  auto-tagged in the shared addBox/addShape helpers by which material they got.
- Textures-off still collapses to the role material (escape hatch); a slot
  override only applies in colored mode.
- Editing a referenced scene material re-dirties the window/door (these systems
  aren't covered by GeometrySystem's scene-material re-dirty).
- New paint capabilities (resolve role from userData.slotId, preview by
  userData.slotId) + capabilities.slots; window/door dropped from the paint
  disabled list. Shared previewSlotByUserData helper.

Defaults unchanged: unpainted windows/doors render exactly as before (the slot
fallback is the existing frame/glass material), so no visual regression.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/nodes/src/shared/slot-paint.ts

@@ -169,6 +169,34 @@ export function previewGeometrySlot(args: PaintPreviewArgs): (() => void) | null
   }
 }
 
+/**
+ * Preview for kinds whose meshes are built by a viewer system (window, door)
+ * and tagged with `userData.slotId` — no `__fromGeometry` marker and no hosted
+ * children to guard against, so it swaps every mesh whose slot matches `role`.
+ */
+export function previewSlotByUserData(args: PaintPreviewArgs): (() => void) | null {
+  const { role, root, material, materialPreset } = args
+  const preview = buildSlotPreviewMaterial(material, materialPreset)
+  if (!preview) return () => {}
+
+  const restores: Array<() => void> = []
+  ;(root as Object3D).traverse((object) => {
+    const mesh = object as Mesh
+    if (!mesh.isMesh) return
+    if ((mesh.userData as { slotId?: string | null }).slotId !== role) return
+    const previous = mesh.material
+    mesh.material = preview
+    restores.push(() => {
+      mesh.material = previous
+    })
+  })
+
+  if (restores.length === 0) return null
+  return () => {
+    for (let index = restores.length - 1; index >= 0; index -= 1) restores[index]?.()
+  }
+}
+
 export type SlotPaintConfig = {
   /** Resolve the slot id for a pointer hit (`null` = not paintable here). */
   resolveRole: (args: PaintResolveArgs) => string | null