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/viewer/src/index.ts

@@ -38,6 +38,10 @@ export { FenceSystem } from './systems/fence/fence-system'
 export { InteractiveSystem } from './systems/interactive/interactive-system'
 export { snapLevelsToTruePositions } from './systems/level/level-utils'
 export { getRoofMaterialArray } from './systems/roof/roof-materials'
+// Slab system follows the wall + fence re-export pattern — composed into
+// the registry-driven slab definition's `def.system`. Removed in Phase 6
+// alongside the legacy slab mount point.
+export { SlabSystem } from './systems/slab/slab-system'
 export { getStairBodyMaterials, getStairRailingMaterial } from './systems/stair/stair-materials'
 export { WallCutout } from './systems/wall/wall-cutout'
 export { getVisibleWallMaterials } from './systems/wall/wall-materials'