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

Same shape as fence — thin renderer + system re-export, capabilities
declared, panel slider-drag fix recipe applied. Pure geometry +
floor-plan ports are later milestones.

Files added (packages/nodes/src/slab/):
 - schema.ts: re-exports SlabNode from core.
 - parametrics.ts: elevation slider only. Polygon + holes edited
   via floor-plan boundary / hole editors, not number inputs.
 - definition.ts: capabilities (no `movable` — slab move is bespoke
   whole-translation through MoveSlabTool that integrates with the
   boundary editor; capability-driven dispatch keeps the legacy mover),
   surfaces.top with elevation-as-height for stacked items, relations
   (hosts: ['item'], cascadeDelete: 'descendants'), toolHints (trace /
   finish / cancel for the placement tool).
 - renderer.tsx: thin placeholder mesh + markDirty on mount + node
   events + cached material via the same getSlabMaterial pattern as
   the legacy renderer (preset apply on shared material instance).
 - system.tsx: re-exports the legacy SlabSystem from viewer.
 - index.ts: barrel.

Files changed:
 - packages/viewer/src/index.ts: exports SlabSystem (already had
   DEFAULT_SLAB_MATERIAL, applyMaterialPresetToMaterials, createMaterial
   from earlier exports).
 - packages/nodes/src/index.ts: appends slabDefinition unconditionally
   to builtinPlugin.nodes.
 - packages/editor/src/components/ui/panels/slab-panel.tsx: applied
   the panel slider-drag fix recipe from plans/editor-node-registry.md
   prophylactically (nodeRef pattern, useScene.getState().updateNode
   inside handler, drop subscribed updateNode dep). Slab's elevation
   slider is the only drag-driven control in the panel — would have
   triggered the same Maximum update depth cascade as wall/fence.

No behavior change. Slab now mounts via the registry path, but the
legacy SlabSystem still does the actual polygon triangulation + hole
CSG work (re-exported, not duplicated).

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

packages/editor/src/components/ui/panels/slab-panel.tsx

@@ -3,7 +3,7 @@
 import { type AnyNode, type SlabNode, useScene } from '@pascal-app/core'
 import { useViewer } from '@pascal-app/viewer'
 import { Edit, Move, Plus, Trash2 } from 'lucide-react'
-import { useCallback, useEffect } from 'react'
+import { useCallback, useEffect, useRef } from 'react'
 import { sfxEmitter } from '../../../lib/sfx-bus'
 import useEditor from '../../../store/use-editor'
 import { ActionButton, ActionGroup } from '../controls/action-button'
@@ -14,7 +14,6 @@ import { PanelWrapper } from './panel-wrapper'
 export function SlabPanel() {
   const selectedId = useViewer((s) => s.selection.selectedIds[0])
   const setSelection = useViewer((s) => s.setSelection)
-  const updateNode = useScene((s) => s.updateNode)
   const editingHole = useEditor((s) => s.editingHole)
   const setEditingHole = useEditor((s) => s.setEditingHole)
   const setMovingNode = useEditor((s) => s.setMovingNode)
@@ -23,12 +22,18 @@ export function SlabPanel() {
     selectedId ? (s.nodes[selectedId as AnyNode['id']] as SlabNode | undefined) : undefined,
   )
 
+  // See "Panel slider-drag fix recipe" in plans/editor-node-registry.md.
+  // Stable handler refs across re-renders so slider drags don't trigger
+  // a Maximum update depth cascade on the panel's SliderControls.
+  const nodeRef = useRef(node)
+  nodeRef.current = node
+
   const handleUpdate = useCallback(
     (updates: Partial<SlabNode>) => {
       if (!selectedId) return
-      updateNode(selectedId as AnyNode['id'], updates)
+      useScene.getState().updateNode(selectedId as AnyNode['id'], updates)
     },
-    [selectedId, updateNode],
+    [selectedId],
   )
 
   const handleClose = useCallback(() => {