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/window/definition.ts

@@ -11,8 +11,10 @@ import { buildRoofWallOpeningCut } from '../shared/roof-wall-opening-cut'
 import { buildWindowFloorplan } from './floorplan'
 import { windowWidthAffordance } from './floorplan-affordances'
 import { windowFloorplanMoveTarget } from './floorplan-move'
+import { windowPaint } from './paint'
 import { windowParametrics } from './parametrics'
 import { WindowNode } from './schema'
+import { windowSlots } from './slots'
 
 const SIDE_HANDLE_OFFSET = 0.24
 const HEIGHT_HANDLE_OFFSET = 0.24
@@ -162,6 +164,10 @@ export const windowDefinition: NodeDefinition<typeof WindowNode> = {
     // `wallId` / `roofSegmentId` are re-derived from the surface under
     // the cursor at preset placement time — see door for the pattern.
     hostRefFields: ['wallId', 'roofSegmentId', 'roofFace'],
+    // Frame / glass slots painted through the registry. The window system tags
+    // each mesh with its `userData.slotId`; paint writes `node.slots`.
+    slots: () => windowSlots(),
+    paint: windowPaint,
   },
 
   parametrics: windowParametrics,