feat(paint-slots): migrate fence onto the unified slot model

Fence joins the node.slots paint model with one paintable 'surface' slot
(merged post/base/rail geometry is a single mesh). Resolution order matches
slab: textures-off joinery role -> node.slots.surface ref -> legacy inline
material/preset -> declared default (library:wood-finewood27). Tags the mesh
userData.slotId='surface' and wires capabilities.slots + paint. Stops
borrowing DEFAULT_STAIR_MATERIAL.

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

packages/nodes/src/fence/paint.ts

@@ -0,0 +1,14 @@
+import type { AnyNode, FenceNode } from '@pascal-app/core'
+import { createSlotPaintCapability, previewGeometrySlot } from '../shared/slot-paint'
+
+export const fencePaint = createSlotPaintCapability({
+  resolveRole: () => 'surface',
+  applyPreview: previewGeometrySlot,
+  legacyEffective: (node: AnyNode) => {
+    const fence = node as FenceNode
+    if (fence.materialPreset || fence.material) {
+      return { material: fence.material, materialPreset: fence.materialPreset }
+    }
+    return null
+  },
+})