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/viewer/src/index.ts

@@ -49,7 +49,7 @@ export { DoorSystem } from './systems/door/door-system'
 // Fence system follows the wall re-export pattern — composed into the
 // registry-driven fence definition's `def.system`. Removed in Phase 6
 // alongside the legacy fence mount point.
-export { FenceSystem } from './systems/fence/fence-system'
+export { FenceSystem, generateFenceGeometry } from './systems/fence/fence-system'
 export { InteractiveSystem } from './systems/interactive/interactive-system'
 // Item systems for the registry-driven item definition. ItemSystem
 // applies attachTo-driven transforms each frame; ItemLightSystem
@@ -61,7 +61,7 @@ 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 { generateSlabGeometry, 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'

packages/viewer/src/systems/fence/fence-system.tsx

@@ -242,7 +242,7 @@ function createFenceParts(fence: FenceNode): FencePart[] {
   return parts
 }
 
-function generateFenceGeometry(fence: FenceNode) {
+export function generateFenceGeometry(fence: FenceNode) {
   const parts = createFenceParts(fence)
   const geometries = parts.map(createFencePartGeometry)