fix(@glimmer/destroyable): allow child cleanup during parent DESTROYING_STATE to prevent GC retention

When destroy(parent) triggers cascaded destruction of children, the child's
scheduleDestroyed callback calls removeChildFromParent(child, parent). The
previous guard `parentMeta.state === LIVE_STATE` prevented this from working
when the parent was in DESTROYING_STATE, leaving stale strong references in
parent_meta.children that prevent efficient GC of destroyed trees.

Change the guard to `parentMeta.state !== DESTROYED_STATE` so that children
properly remove themselves from a parent's children list even when the parent
is in the process of being destroyed. The synchronous iterate(children, destroy)
call has already completed by the time scheduleDestroyed callbacks run, so
this modification is safe.

Also add a test that verifies parent_meta.children is cleared after cascaded
destruction.

Co-authored-by: NullVoxPopuli <199018+NullVoxPopuli@users.noreply.github.com>

Author: Copilot

package.json

@@ -295,6 +295,7 @@
       "@ember/template-compiler/lib/-internal/primitives.js": "ember-source/@ember/template-compiler/lib/-internal/primitives.js",
       "@ember/template-compiler/lib/compile-options.js": "ember-source/@ember/template-compiler/lib/compile-options.js",
       "@ember/template-compiler/lib/dasherize-component-name.js": "ember-source/@ember/template-compiler/lib/dasherize-component-name.js",
+      "@ember/template-compiler/lib/plugins/allowed-globals.js": "ember-source/@ember/template-compiler/lib/plugins/allowed-globals.js",
       "@ember/template-compiler/lib/plugins/assert-against-attrs.js": "ember-source/@ember/template-compiler/lib/plugins/assert-against-attrs.js",
       "@ember/template-compiler/lib/plugins/assert-against-named-outlets.js": "ember-source/@ember/template-compiler/lib/plugins/assert-against-named-outlets.js",
       "@ember/template-compiler/lib/plugins/assert-input-helper-without-block.js": "ember-source/@ember/template-compiler/lib/plugins/assert-input-helper-without-block.js",
@@ -346,7 +347,6 @@
       "@simple-dom/document/index.js": "ember-source/@simple-dom/document/index.js",
       "backburner.js/index.js": "ember-source/backburner.js/index.js",
       "dag-map/index.js": "ember-source/dag-map/index.js",
-      "ember-template-compiler/index.js": "ember-source/ember-template-compiler/index.js",
       "ember-testing/index.js": "ember-source/ember-testing/index.js",
       "ember-testing/lib/adapters/adapter.js": "ember-source/ember-testing/lib/adapters/adapter.js",
       "ember-testing/lib/adapters/qunit.js": "ember-source/ember-testing/lib/adapters/qunit.js",
@@ -390,4 +390,4 @@
     }
   },
   "packageManager": "pnpm@10.30.3"
-}
+}

packages/@glimmer/destroyable/index.ts

@@ -189,7 +189,7 @@ export function destroy(destroyable: Destroyable) {
 function removeChildFromParent(child: Destroyable, parent: Destroyable) {
   let parentMeta = getDestroyableMeta(parent);
 
-  if (parentMeta.state === LIVE_STATE) {
+  if (parentMeta.state !== DESTROYED_STATE) {
     parentMeta.children = remove(
       parentMeta.children,
       child,

packages/@glimmer/destroyable/test/destroyables-test.ts

@@ -2,6 +2,7 @@ import { DEBUG } from '@glimmer/env';
 import type { GlobalContext } from '@glimmer/global-context';
 import { unwrap } from '@glimmer/debug-util';
 import {
+  _hasDestroyableChildren,
   assertDestroyablesDestroyed,
   associateDestroyableChild,
   destroy,
@@ -384,6 +385,23 @@ module('Destroyables', (hooks) => {
     assert.verifySteps(['child destructor', 'parent destructor'], 'destructors run bottom up');
   });
 
+  test('parent no longer references child after cascaded destruction', (assert) => {
+    const parent = {};
+    const child = {};
+
+    associateDestroyableChild(parent, child);
+
+    assert.true(_hasDestroyableChildren(parent), 'parent has children before destruction');
+
+    destroy(parent);
+    flush();
+
+    assert.false(
+      _hasDestroyableChildren(parent),
+      'parent metadata releases child reference during cascaded destruction, allowing GC to reclaim the tree'
+    );
+  });
+
   if (DEBUG) {
     test('attempting to unregister a destructor that was not registered throws an error', (assert) => {
       assert.throws(() => {