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/door/slots.ts

@@ -0,0 +1,19 @@
+import type { SlotDeclaration } from '@pascal-app/core'
+
+export type DoorSlotId = 'panel' | 'glass'
+
+// Picker swatches. Rendering falls back to the live body/glass defaults (which
+// already track shading + theme), so these are just the indicator colours.
+const PANEL_DEFAULT = '#f2f0ed'
+const GLASS_DEFAULT = '#87ceeb'
+
+/**
+ * A door exposes two paintable slots: `panel` (the door body — frame casing +
+ * leaf) and `glass`. The opening reveal keeps its own material.
+ */
+export function doorSlots(): SlotDeclaration[] {
+  return [
+    { slotId: 'panel', label: 'Panel', default: PANEL_DEFAULT },
+    { slotId: 'glass', label: 'Glass', default: GLASS_DEFAULT },
+  ]
+}