Phase 5 depth-first: spawn C, fence B+C, slab B+C, ceiling C

Depth-first session: drive registered kinds through Stage B (pure
def.geometry, drop system re-export) and Stage C (def.floorplan,
short-circuit legacy inline rendering in floorplan-panel.tsx).

spawn → C
 - buildSpawnFloorplan wired on definition (was written but deferred
   to avoid double-render).
 - floorplan-panel.tsx's floorplanSpawnEntries useMemo short-circuits
   to [] when nodeRegistry.has('spawn').

fence → B
 - generateFenceGeometry exported from viewer; buildFenceGeometry
   wraps it in a Group+Mesh with DEFAULT_STAIR_MATERIAL.
 - def.geometry set; renderer + system fields dropped.
 - Deleted nodes/src/fence/{renderer.tsx,system.tsx}.

fence → C
 - buildFenceFloorplan: polyline along centerline (sampled for curved
   fences via sampleWallCenterline from core). Stroke width = node.thickness.
 - floorplan-panel.tsx's floorplanFenceEntries short-circuits.

slab → B
 - generateSlabGeometry exported from viewer; buildSlabGeometry wraps
   it in a Group+Mesh + cached material (preset / custom / default
   pattern preserved from legacy renderer).
 - def.geometry set; renderer + system fields dropped.
 - Deleted nodes/src/slab/{renderer.tsx,system.tsx}.

slab → C
 - buildSlabFloorplan: SVG path with outer polygon + hole subpaths
   (uses getRenderableSlabPolygon from core for wall-clipping parity).
 - floorplan-panel.tsx's slabPolygons short-circuits.

ceiling → B INTENTIONALLY SKIPPED
 - Ceiling renderer renders React children (hosted items) + uses TSL
   shader materials + named meshes that other systems poke
   (getObjectByName('ceiling-grid')). Pure def.geometry can't preserve
   that. Ceiling keeps def.renderer (the custom escape hatch) — same
   pattern item uses. Documented in ceiling/definition.ts.

ceiling → C
 - buildCeilingFloorplan: dashed-outline path with hole subpaths
   (visually distinct from slab since ceilings are above).
 - floorplan-panel.tsx's ceilingPolygons short-circuits.

Per-kind progress after this session:
 - shelf: B ✅ C ✅ (Stage E since brand-new)
 - spawn: A ✅ C ✅
 - wall: A ✅ (B blocked on ctx.levelData design)
 - fence: A ✅ B ✅ C ✅
 - slab: A ✅ B ✅ C ✅
 - ceiling: A ✅ C ✅ (B intentionally not applicable)
 - door / window / item: A ✅ (B+C pending in future sessions)

Known test issue: `bun test` in packages/nodes fails to load
`three-bvh-csg` through the viewer's transitive imports (UMD/ESM
mismatch in Bun's test runner). The Next.js editor build works fine
because it bundles differently. Fix requires either dynamic imports
(breaks sync def.geometry contract) or test env config — deferred.
Other tests (schema, geometry, parity) pass.

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

packages/nodes/src/fence/geometry.ts

@@ -0,0 +1,29 @@
+import { DEFAULT_STAIR_MATERIAL, generateFenceGeometry } from '@pascal-app/viewer'
+import { Group, Mesh } from 'three'
+import type { FenceNode } from './schema'
+
+/**
+ * Stage B builder for fence. Reuses the legacy `generateFenceGeometry`
+ * (pure function from viewer that returns a merged BufferGeometry of
+ * posts + base + top rail + curve spans) and wraps it in a Mesh-in-Group
+ * shape the generic `<GeometrySystem>` expects.
+ *
+ * Material is a single shared reference — fences look the same regardless
+ * of instance, so we don't clone per node. If per-fence material
+ * customization lands later (color picker on the panel maps to a real
+ * material), this becomes a per-node lookup.
+ *
+ * Phase 6 cleanup moves the 280 lines of geometry math out of the
+ * legacy `viewer/src/systems/fence/fence-system.tsx` into this folder
+ * once the legacy system file is deleted. Until then `generateFenceGeometry`
+ * is publicly re-exported from viewer.
+ */
+export function buildFenceGeometry(node: FenceNode): Group {
+  const group = new Group()
+  const geometry = generateFenceGeometry(node)
+  const mesh = new Mesh(geometry, DEFAULT_STAIR_MATERIAL)
+  mesh.castShadow = true
+  mesh.receiveShadow = true
+  group.add(mesh)
+  return group
+}