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/index.ts

@@ -0,0 +1,3 @@
+export { fenceDefinition } from './definition'
+export { isFenceRegistryEnabled } from './feature-flag'
+export { FenceNode } from './schema'