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/viewer/src/index.ts

@@ -2,6 +2,12 @@
 // renderers (wall renders doors/windows, slab renders hosted items).
 // Public so registry-driven kinds can compose children without reaching
 // into viewer's internal paths.
+
+// Ceiling internals re-exported for the registry-driven ceiling definition.
+// The ceiling renderer uses TSL shaders for a grid pattern — too much code
+// to duplicate at Stage A. Wrap-exported here, ported into the nodes
+// folder in a later stage.
+export { CeilingRenderer } from './components/renderers/ceiling/ceiling-renderer'
 export { NodeRenderer } from './components/renderers/node-renderer'
 export { default as Viewer } from './components/viewer'
 export type { HoverStyle, HoverStyles } from './components/viewer/post-processing'
@@ -31,6 +37,7 @@ export {
 } from './lib/materials'
 export { mergedOutline } from './lib/merged-outline-node'
 export { default as useViewer } from './store/use-viewer'
+export { CeilingSystem } from './systems/ceiling/ceiling-system'
 // Fence system follows the wall re-export pattern — composed into the
 // registry-driven fence definition's `def.system`. Removed in Phase 6
 // alongside the legacy fence mount point.