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/nodes/src/slab/definition.ts

@@ -0,0 +1,88 @@
+import type { NodeDefinition } from '@pascal-app/core'
+import { slabParametrics } from './parametrics'
+import { SlabNode } from './schema'
+
+/**
+ * Slab — Phase 5 batch kind, polygon-based.
+ *
+ * Capabilities:
+ *  - **No `movable`**: slab's "move" today is whole-slab translation via
+ *    legacy `MoveSlabTool`, which integrates with the floor-plan boundary /
+ *    hole editors. Per the capability-driven dispatch rule, omitting
+ *    `movable` keeps the legacy mover (preserves polygon-aware behavior).
+ *    Migration to the generic mover is possible in a later milestone if
+ *    the legacy mover proves equivalent.
+ *  - **`surfaces.top`**: items host on the slab top at `elevation`.
+ *  - `selectable`, `duplicable`, `deletable` standard.
+ *
+ * Relations:
+ *  - `hosts: ['item']` — items mount on the slab top.
+ *  - `cascadeDelete: 'descendants'` — deleting a slab removes hosted items.
+ *
+ * Renderer + system: thin renderer + re-export of the legacy `SlabSystem`.
+ * Same shape as wall / fence runtime port.
+ *
+ * Tool field absent: slab has 3 tools (slab-tool, boundary-editor, hole-
+ * editor) wired through editor state, not registry dispatch.
+ */
+export const slabDefinition: NodeDefinition<typeof SlabNode> = {
+  kind: 'slab',
+  schemaVersion: 1,
+  schema: SlabNode,
+  category: 'structure',
+
+  defaults: () => ({
+    object: 'node',
+    parentId: null,
+    visible: true,
+    metadata: {},
+    polygon: [],
+    holes: [],
+    holeMetadata: [],
+    elevation: 0.05,
+    autoFromWalls: false,
+  }),
+
+  capabilities: {
+    selectable: { hitVolume: 'bbox' },
+    surfaces: {
+      top: { height: (n) => (n as SlabNode).elevation },
+    },
+    duplicable: true,
+    deletable: true,
+  },
+
+  relations: {
+    hosts: ['item'],
+    cascadeDelete: 'descendants',
+  },
+
+  parametrics: slabParametrics,
+
+  renderer: {
+    kind: 'parametric',
+    module: () => import('./renderer'),
+  },
+  system: {
+    module: () => import('./system'),
+    priority: 4,
+  },
+
+  toolHints: [
+    { key: 'Left click', label: 'Trace slab outline' },
+    { key: 'Enter', label: 'Finish slab' },
+    { key: 'Esc', label: 'Cancel' },
+  ],
+
+  presentation: {
+    label: 'Slab',
+    description: 'A polygon-bounded floor surface that hosts items on top.',
+    icon: { kind: 'iconify', name: 'lucide:square' },
+    paletteSection: 'structure',
+    paletteOrder: 30,
+  },
+
+  mcp: {
+    description: 'A polygon-bounded slab (floor) with optional cutout holes.',
+  },
+}