Phase 5 batch kind: ceiling migrates to registry (always-on)

Structurally identical to slab. Stage A migration: registers the kind,
wraps the legacy renderer + system, applies the panel slider-drag fix
recipe.

Files added (packages/nodes/src/ceiling/):
 - schema.ts: re-exports CeilingNode from core.
 - parametrics.ts: height slider only. Polygon + holes via floor-plan
   editors.
 - definition.ts: capabilities (no `movable`, `surfaces.top` mapped to
   `height`), relations (hosts: ['item'] for ceiling-mounted lights /
   fans, cascadeDelete: 'descendants'), toolHints, parametrics.
 - renderer.tsx: wrap-export of legacy CeilingRenderer. The legacy
   renderer uses TSL shader code for grid-line patterns (~100 lines);
   not worth duplicating at Stage A. Per-stage migration plan in
   plans/editor-node-registry.md moves the renderer body into this
   folder at Stage B/F.
 - system.tsx: re-exports legacy CeilingSystem.
 - index.ts: barrel.

Files changed:
 - packages/viewer/src/index.ts: new public exports for CeilingRenderer
   + CeilingSystem.
 - packages/nodes/src/index.ts: appends ceilingDefinition.
 - packages/editor/src/components/ui/panels/ceiling-panel.tsx: panel
   slider-drag fix recipe applied (nodeRef pattern, useScene.getState()
   inside handler, drop subscribed updateNode dep) so the height
   slider doesn't trigger the same cascade fence + wall + slab fixed.

Phase 5 progress: shelf ✅, spawn ✅, wall ✅, fence ✅, slab ✅, ceiling ✅.
Six kinds on the registry. Door / window / item / stair / roof / zone
follow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/src/index.ts

@@ -1,4 +1,5 @@
 import type { AnyNodeDefinition, Plugin } from '@pascal-app/core'
+import { ceilingDefinition } from './ceiling'
 import { fenceDefinition } from './fence'
 import { shelfDefinition } from './shelf'
 import { slabDefinition } from './slab'
@@ -32,9 +33,11 @@ export const builtinPlugin: Plugin = {
     wallDefinition as unknown as AnyNodeDefinition,
     fenceDefinition as unknown as AnyNodeDefinition,
     slabDefinition as unknown as AnyNodeDefinition,
+    ceilingDefinition as unknown as AnyNodeDefinition,
   ],
 }
 
+export { ceilingDefinition } from './ceiling'
 export { fenceDefinition } from './fence'
 export { shelfDefinition } from './shelf'
 export { slabDefinition } from './slab'