feat(editor): roof features in standalone Build tab + thumbnail flag fix

- registry: getRoofAccessoryKinds() enumerates kinds declaring the
  roofAccessory capability, in deterministic builtin-list order (mirrors
  getSelectableKinds); exported from the registry barrel.
- apps/editor build-tab: a "Features" group under the Roof tile, discovered
  from the registry (no DB) and activating each kind's roof-attach tool —
  parity with the community editor's roof features.
- site-boundary-editor: hide the flag handle group around thumbnail captures
  (they render on SCENE_LAYER so the thumbnail camera can't layer-filter them),
  matching handle-arrow.tsx — keeps flags out of preset/snapshot thumbnails.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/core/src/registry/index.ts

@@ -17,6 +17,7 @@ export type {
 export {
   discoverPlugins,
   getHostRefFields,
+  getRoofAccessoryKinds,
   getSelectableKinds,
   isDrawnViaTool,
   isDrawnViaToolKind,

packages/core/src/registry/registry.ts

@@ -114,6 +114,28 @@ export function isRegistrySelectable(kind: string): boolean {
   return nodeRegistry.get(kind)?.capabilities.selectable !== undefined
 }
 
+/**
+ * Kinds whose definition declares the `roofAccessory` capability — the roof
+ * accessories (dormer, chimney, vents, gutter, …) that mount onto a roof
+ * segment via their own attach tool. Lets host UIs surface a "Features" group
+ * under the roof category without hardcoding the kind list (the standalone
+ * editor's Build tab; the roof inspector's add menu). Returned in builtin
+ * registration order (`packages/nodes/src/index.ts`), which is deterministic.
+ *
+ * Call at render time, not module-import time: the registry is populated by
+ * the host's bootstrap (`loadPlugin`), so a top-level `const` would race it
+ * and see an empty registry.
+ */
+export function getRoofAccessoryKinds(): string[] {
+  const result: string[] = []
+  for (const [kind, def] of nodeRegistry.entries()) {
+    if (def.capabilities.roofAccessory !== undefined) {
+      result.push(kind)
+    }
+  }
+  return result
+}
+
 /**
  * Kinds whose `def.floorplanScope` matches the requested scope. Used by
  * `FloorplanRegistryLayer` to discover building-scoped kinds (e.g.