feat(paint-slots): round-2 feedback — fence 2 slots + UVs, roof defaults, stair body

- fence: split into two paint slots — panel (posts/base/infill, default charcoal)
  and rail (cap, default wood-finewood27) — as separate meshes with userData.slotId.
  Fix applyFenceUVs to continuous world-space 1 UV unit = 1 m (drop the per-part
  min origin that broke tiling across parts). New generateFenceSlotGeometries.
- roof: real catalog defaults for the segment surfaces via getRoofMaterialArray
  (the actual default path): wall/trim concrete-plate (matches walls), deck +
  soffit soft-white, shingle terracotta; textures-off role escape hatch kept.
  Align nodes getRoofMaterials no-parent fallback to match.
- stair: body slot default -> preset-lightgrey.
- (biome formatting normalization of the round-1 merged renderer files rides along.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/viewer/src/index.ts

@@ -97,7 +97,11 @@ export { ElevatorInteractionSystem } from './systems/elevator/elevator-interacti
 // 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, generateFenceGeometry } from './systems/fence/fence-system'
+export {
+  FenceSystem,
+  generateFenceGeometry,
+  generateFenceSlotGeometries,
+} from './systems/fence/fence-system'
 // Generic floor-elevation system. Lifts the rendered mesh of any kind
 // whose definition declares `capabilities.floorPlaced` by the slab
 // elevation under its footprint. Replaces the per-kind elevation block