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/slab/floorplan.ts

@@ -0,0 +1,52 @@
+import {
+  type FloorplanGeometry,
+  type FloorplanPoint,
+  getRenderableSlabPolygon,
+  type SlabNode,
+} from '@pascal-app/core'
+
+/**
+ * Stage C floor-plan builder for slab. Renders the slab polygon as a
+ * filled path with holes cut out.
+ *
+ * Uses `getRenderableSlabPolygon` (the same helper the legacy
+ * floorplan-panel.tsx uses) to compute the visual polygon — accounts
+ * for wall-clipping when a slab is auto-generated from walls.
+ */
+export function buildSlabFloorplan(node: SlabNode): FloorplanGeometry | null {
+  const polygon = node.polygon
+  if (!polygon || polygon.length < 3) return null
+
+  const visualPolygon = getRenderableSlabPolygon(node)
+  if (!visualPolygon || visualPolygon.length < 3) return null
+
+  const outer: FloorplanPoint[] = visualPolygon.map(([x, z]) => [x, z] as FloorplanPoint)
+
+  // SVG path with outer ring + hole subpaths. Each subpath uses M/L
+  // commands + Z to close. Holes follow the outer ring; FloorplanGeometry
+  // 'path' kind supports this natively (renderer passes the `d` string
+  // straight to the SVG <path>).
+  const segments: string[] = []
+  const ring = (points: FloorplanPoint[]) => {
+    const [first, ...rest] = points
+    if (!first) return ''
+    return [`M ${first[0]} ${first[1]}`, ...rest.map(([x, y]) => `L ${x} ${y}`), 'Z'].join(' ')
+  }
+  segments.push(ring(outer))
+
+  const holes = node.holes ?? []
+  for (const hole of holes) {
+    if (hole.length < 3) continue
+    const holePts: FloorplanPoint[] = hole.map(([x, z]) => [x, z] as FloorplanPoint)
+    segments.push(ring(holePts))
+  }
+
+  return {
+    kind: 'path',
+    d: segments.join(' '),
+    fill: '#cbd5e1',
+    stroke: '#475569',
+    strokeWidth: 0.03,
+    opacity: 0.85,
+  }
+}