Phase 5 first batch kind: fence migrates to registry behind feature flag

Same shape as wall milestone B — thin renderer + system re-export, no
geometry / floor-plan / tool ports yet (later milestones). Feature flag
NEXT_PUBLIC_USE_REGISTRY_FOR_FENCE gates the dispatch flip.

Files added (packages/nodes/src/fence/):
 - schema.ts: re-exports FenceNode from core.
 - parametrics.ts: dimensions / posts / style fields for the auto-
   inspector. Endpoints + curveOffset edited via tools, not in
   parametrics.
 - feature-flag.ts: mirrors the wall flag pattern.
 - definition.ts: capabilities (snappable + surfaces sides +
   selectable + duplicable + deletable), relations (linkedBy
   endpoint-match, no hosts, no affectsSpatial — matches legacy),
   parametrics, renderer, system, toolHints (Left click / Shift /
   Esc — fence has no helper file today so this adds a panel where
   there wasn't one). Tool field absent: fence has 4 tools (build,
   curve, move, move-endpoint) wired through editor state, not the
   registry dispatch — they keep running unchanged.
 - renderer.tsx: thin placeholder mesh + markDirty on mount + node
   events + DEFAULT_STAIR_MATERIAL (matches legacy material reuse).
   Verification log fires once on first mount.
 - system.tsx: re-exports the legacy FenceSystem from viewer.
   Verification log on mount/unmount confirms the bundle activates.
 - index.ts: barrel.

Files changed:
 - packages/viewer/src/index.ts: new exports for FenceSystem and
   DEFAULT_STAIR_MATERIAL so the @pascal-app/nodes bundle can
   compose them without reaching into viewer internals.
 - packages/viewer/src/components/renderers/fence/fence-renderer.tsx:
   paired one-shot legacy verification log so the dispatch path is
   unambiguous from the browser console.
 - packages/nodes/src/index.ts: conditional fenceEntries appended to
   builtinPlugin.nodes based on isFenceRegistryEnabled. With the flag
   off (default), behavior is unchanged; with it on, Phase 0 shims
   switch fence to the registry path — legacy <FenceRenderer> and
   <LegacySystem kind="fence"><FenceSystem /></LegacySystem> short-
   circuit, the bundled system.tsx re-mounts FenceSystem via
   RegisteredSystems, and the new renderer takes over the dispatch.

No behavior change with the flag off. With it on, behavior should be
byte-identical (same FenceSystem code, same priority, same geometry
path).

Phase 5 batch order continues with slab / ceiling / door / window /
item / etc. as flagged migrations after fence parity signs off.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/src/fence/definition.ts

@@ -0,0 +1,102 @@
+import type { NodeDefinition } from '@pascal-app/core'
+import { fenceParametrics } from './parametrics'
+import { FenceNode } from './schema'
+
+/**
+ * Fence — the first Phase 5 batch-migration kind.
+ *
+ * What this definition encodes:
+ *  - **Capabilities**: snappable (other walls/fences/items snap to it),
+ *    surfaces (front + back faces host items), selectable, duplicable,
+ *    deletable. No `movable` capability — fence move is bespoke
+ *    endpoint-drag, same shape as wall (handled by legacy MoveFenceTool
+ *    until the affordance port).
+ *  - **Relations**: `linkedBy: 'endpoint-match'` for fence-corner cascade.
+ *    No `hosts` field — doors/windows don't mount on fences. `affectsSpatial`
+ *    omitted: moving a fence doesn't dirty slabs/zones in the legacy
+ *    behavior, so the registry stays parity-equivalent until we
+ *    explicitly add the cascade (separate decision).
+ *  - **Parametrics**: dimensions, posts, style — see `./parametrics.ts`.
+ *  - **toolHints**: placement panel hints for the fence-build tool.
+ *  - **Renderer + system**: thin placeholder mesh + re-export of the
+ *    legacy `FenceSystem`. Same shape as wall milestone B; future Phase 5+
+ *    extracts the pure geometry function and migrates to `def.geometry`.
+ *
+ * Tool field stays absent: fence has 4 separate tools (build, curve,
+ * move, move-endpoint) wired through editor state, not the registry
+ * tool dispatch. They keep running unchanged until the affordance port.
+ *
+ * Migration is gated by `feature-flag.ts`
+ * (env: `NEXT_PUBLIC_USE_REGISTRY_FOR_FENCE`). See
+ * `plans/editor-node-registry.md#phase-5` for the batch order.
+ */
+export const fenceDefinition: NodeDefinition<typeof FenceNode> = {
+  kind: 'fence',
+  schemaVersion: 1,
+  schema: FenceNode,
+  category: 'structure',
+
+  defaults: () => ({
+    object: 'node',
+    parentId: null,
+    visible: true,
+    metadata: {},
+    start: [0, 0],
+    end: [3, 0],
+    height: 1.8,
+    thickness: 0.08,
+    baseHeight: 0.22,
+    postSpacing: 2,
+    postSize: 0.1,
+    topRailHeight: 0.04,
+    groundClearance: 0,
+    edgeInset: 0.015,
+    baseStyle: 'grounded',
+    showInfill: true,
+    color: '#ffffff',
+    style: 'slat',
+  }),
+
+  capabilities: {
+    selectable: { hitVolume: 'bbox' },
+    surfaces: { sides: { faces: 'all' } },
+    duplicable: true,
+    deletable: true,
+  },
+
+  relations: {
+    linkedBy: 'endpoint-match',
+    cascadeDelete: 'none',
+  },
+
+  parametrics: fenceParametrics,
+
+  renderer: {
+    kind: 'parametric',
+    module: () => import('./renderer'),
+  },
+  system: {
+    module: () => import('./system'),
+    // Same frame priority as the legacy FenceSystem (4 — runs after door/
+    // window animations at 2-3, before zone/level systems at 6+).
+    priority: 4,
+  },
+
+  toolHints: [
+    { key: 'Left click', label: 'Set fence start / end' },
+    { key: 'Shift', label: 'Allow non-45° angles' },
+    { key: 'Esc', label: 'Cancel' },
+  ],
+
+  presentation: {
+    label: 'Fence',
+    description: 'A straight or curved fence segment with configurable posts and infill.',
+    icon: { kind: 'iconify', name: 'lucide:fence' },
+    paletteSection: 'structure',
+    paletteOrder: 20,
+  },
+
+  mcp: {
+    description: 'A fence segment defined by start + end points, with optional curve sagitta.',
+  },
+}