Selection: registry-driven, drop spawn flag, restore green color

Two concerns from the spike:

1) Selection / floating-action-menu had hardcoded kind lists scattered
   across 4 files. Adding 'shelf' to each one per migration was the
   wrong abstraction — the user's question "did you make it generic
   from the noderegistry?" was the right one. Done now.

   Added to @pascal-app/core/registry:
   - getSelectableKinds(): string[] — returns all registered kinds
     whose definition declares `capabilities.selectable`.
   - isRegistrySelectable(kind): boolean — predicate for OR-chains.

   Refactored hardcoded sites to merge registry kinds at runtime,
   keeping legacy hardcoded lists intact so existing kinds keep
   working unchanged:
   - editor SelectionManager: 4 subscription loops (enter/leave/click)
     + structure.isValid + getSelectionTarget — all augment with
     registry kinds. Phase 6 deletes the hardcoded lists.
   - viewer SelectionManager: subscription loop + SelectableNodeType
     broadened with `(string & {})` to accept registry kinds.
   - floating-action-menu: ALLOWED_TYPES OR'd with isRegistrySelectable.
   - Removed the manually-added 'shelf' entries from previous commit
     857ddd4; they were redundant once the registry-driven path landed.

   Future built-in nodes that declare `capabilities.selectable` get
   click-selection + hover + the floating action menu (move/delete
   icons) for free, no editing of these 4 files.

2) Spawn parity is signed off. Drop the
   NEXT_PUBLIC_USE_REGISTRY_FOR_SPAWN flag entirely; spawn registers
   unconditionally in builtinPlugin.nodes. Restored SPAWN_COLOR to
   the original #22c55e green (was #ef4444 red as a Phase 2
   verification marker).

Pre-existing typecheck errors in editor (ceiling/fence/slab tree-node,
scene.ts buildingId) are unchanged.

630 tests still pass.

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

packages/core/src/registry/index.ts

@@ -1,4 +1,10 @@
-export { loadPlugin, nodeRegistry, registerNode } from './registry'
+export {
+  getSelectableKinds,
+  isRegistrySelectable,
+  loadPlugin,
+  nodeRegistry,
+  registerNode,
+} from './registry'
 export {
   type CascadeContext,
   type ChildQuery,

packages/core/src/registry/registry.ts

@@ -57,6 +57,34 @@ export function registerNode(def: AnyNodeDefinition): void {
   nodeRegistry._register(def)
 }
 
+/**
+ * Returns the set of registered kinds whose definition declares the
+ * `selectable` capability. Callers that maintain hardcoded "selectable kinds"
+ * lists (SelectionManager, FloatingActionMenu) should concat this with their
+ * legacy entries instead of editing the hardcoded list per migration.
+ *
+ * Phase 6 deletes the hardcoded lists entirely and uses this function as the
+ * single source of truth. For now it's additive over the legacy lists so the
+ * existing kinds keep working unchanged.
+ */
+export function getSelectableKinds(): string[] {
+  const result: string[] = []
+  for (const [kind, def] of nodeRegistry.entries()) {
+    if (def.capabilities.selectable !== undefined) {
+      result.push(kind)
+    }
+  }
+  return result
+}
+
+/**
+ * Returns true when the kind is declared selectable in the registry. Use
+ * in expression chains like `if (node.type === 'wall' || isRegistrySelectable(node.type))`.
+ */
+export function isRegistrySelectable(kind: string): boolean {
+  return nodeRegistry.get(kind)?.capabilities.selectable !== undefined
+}
+
 export async function loadPlugin(plugin: Plugin): Promise<void> {
   if (plugin.apiVersion !== HOST_API_VERSION) {
     throw new Error(