Wall Phase 3 milestone B: runtime port behind feature flag

Brings the wall kind onto the registry path when
NEXT_PUBLIC_USE_REGISTRY_FOR_WALL=true; default-off keeps wall on its
legacy path unchanged.

Files added:

- nodes/src/wall/renderer.tsx — thin placeholder-mesh mount point.
  Identical pattern to the legacy WallRenderer: registers ref via
  useRegistry, marks dirty on mount, renders hosted children
  recursively via NodeRenderer. The legacy WallSystem fills geometry
  on the next frame regardless of which mount path is active.

- nodes/src/wall/system.tsx — a bundle component that renders
  <WallSystem /> + <WallCutout /> (both re-exported from viewer).
  Registered via def.system with priority 4 to mirror the legacy
  WallSystem's useFrame priority. Zero logic duplication — the
  ~970 lines of CSG/mitering/cutaway code stays in viewer.

Files changed:

- packages/viewer/src/index.ts — new exports for WallSystem, WallCutout,
  and NodeRenderer. The first two so the registry-driven system bundle
  can compose them; NodeRenderer so any parent kind (wall, slab,
  ceiling, building) can recursively render hosted children without
  reaching into viewer internals.

- nodes/src/wall/definition.ts — adds renderer + system fields. Tool
  field stays absent (wall placement / endpoint drag remain bespoke
  for now; the affordance port is a later milestone).

- nodes/src/index.ts — conditionally appends wallDefinition to
  builtinPlugin.nodes based on isWallRegistryEnabled(). With the flag
  off, the array is identical to before this commit; with it on,
  Phase 0 dispatch shims switch wall to the registry path:
    * <LegacySystem kind="wall"> around WallSystem returns null
    * <LegacySystem kind="wall"> around WallCutout returns null
    * <NodeRenderer> takes the registry-first branch and mounts the
      new renderer instead of the legacy switch case for 'wall'
    * RegisteredSystems mounts the new system bundle, which re-mounts
      the same WallSystem + WallCutout components from viewer

No behavior change with the flag off. With the flag on, behavior should
be byte-identical (same components, same priority, same geometry path).
Manual verification next: place walls, t-junctions, walls-with-doors
with the flag toggled both ways; confirm visual + interactive parity.

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

packages/nodes/src/wall/definition.ts

@@ -90,9 +90,28 @@ export const wallDefinition: NodeDefinition<typeof WallNode> = {
 
   parametrics: wallParametrics,
 
-  // No `geometry` / `renderer` / `system` / `tool` fields yet — see file
-  // header. Adding them registers wall via `builtinPlugin.nodes` and flips
-  // the dispatch shims; do that in milestone B once the runtime port lands.
+  // Wall's renderer is the thin placeholder-mesh mount point from milestone
+  // B; the system bundle composes the legacy `WallSystem` + `WallCutout`
+  // re-exported from viewer (so we don't duplicate ~970 lines of CSG /
+  // mitering / cutaway logic just to swap the dispatch). The legacy
+  // mount in `<LegacySystem kind="wall">` short-circuits the moment
+  // `nodeRegistry.has('wall')` is true.
+  //
+  // No `tool` yet — wall placement / endpoint drag / curve drag remain
+  // bespoke until the affordance port lands. Phase 0 shims keep the legacy
+  // wall tool running while wall is registered (it's not wired through the
+  // registry tool dispatch).
+  renderer: {
+    kind: 'parametric',
+    module: () => import('./renderer'),
+  },
+  system: {
+    module: () => import('./system'),
+    // Priority 4 mirrors the legacy WallSystem's useFrame priority — keeps
+    // miter cascade running after door/window animation systems (priority 2)
+    // but before zone/level systems that read wall positions.
+    priority: 4,
+  },
 
   presentation: {
     label: 'Wall',