fix(plugin-validation): allow parent-bundled extensions

Accept reusable canvas sources that are referenced by an existing parent plugin, matching the post-#2546 scaffolding guidance, while continuing to reject orphaned sources. Cover parent-only, standalone, and orphaned registrations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 52be9c67-3ae4-4610-93d0-fe0b7ab95ccb

eng/validate-plugins.test.mjs

@@ -0,0 +1,36 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { isReusableExtensionRegistered } from "./validate-plugins.mjs";
+
+test("accepts a reusable extension bundled only by a parent plugin", () => {
+  assert.equal(
+    isReusableExtensionRegistered(
+      "daily-focus-board",
+      new Set(["ember"]),
+      new Set(["daily-focus-board"])
+    ),
+    true
+  );
+});
+
+test("accepts a same-named standalone extension plugin", () => {
+  assert.equal(
+    isReusableExtensionRegistered(
+      "daily-focus-board",
+      new Set(["daily-focus-board"]),
+      new Set()
+    ),
+    true
+  );
+});
+
+test("rejects an orphaned reusable extension", () => {
+  assert.equal(
+    isReusableExtensionRegistered(
+      "daily-focus-board",
+      new Set(["ember"]),
+      new Set()
+    ),
+    false
+  );
+});