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/slots.ts

@@ -0,0 +1,9 @@
+import type { SlotDeclaration } from '@pascal-app/core'
+
+export type FenceSlotId = 'surface'
+
+export const FENCE_SLOT_DEFAULT = 'library:wood-finewood27'
+
+export function fenceSlots(): SlotDeclaration[] {
+  return [{ slotId: 'surface', label: 'Surface', default: FENCE_SLOT_DEFAULT }]
+}