refactor: address review — rename activateSiblingTimelines, opts arg, FIXME tracking

- Rename activateNestedChildTimelines → activateSiblingTimelines (matches player.ts)
- Use tl.play() instead of tl.paused(false) for consistency
- Convert positional activateChildren boolean to { activateChildren } opts
- Add FIXME(#969) to divergence test with tracking issue link
- Add [id="intro"] no-rewrite boundary test
- Add comment about deliberate no-restore behavior in render-seek path

packages/core/src/compiler/compositionScoping.test.ts

@@ -513,6 +513,17 @@ window.__afterTimeline = window.__timelines.scene;
     expect(scoped).not.toMatch(/#intro\b/);
   });
 
+  it('does not rewrite [id="intro"] attribute selectors', () => {
+    // The function only targets #intro hash selectors, not [id="intro"] attribute selectors
+    const result = scopeCssToComposition(
+      '[id="intro"] .title { color: red; }',
+      "intro",
+      undefined,
+      "intro",
+    );
+    expect(result).toContain('[id="intro"]');
+  });
+
   it("wraps scripts with authored root id normalization for #id GSAP selectors", () => {
     const { document } = parseHTML(`
       <div data-composition-id="intro">

packages/core/src/compiler/inlineSubCompositions.test.ts

@@ -147,6 +147,8 @@ describe("inlineSubCompositions – #ID selector scoping divergence", () => {
    * Proper fix (follow-up): make the producer path add data-hf-authored-id
    * to the host element when the inner root has an id attribute.
    */
+  // FIXME(#969): flip these assertions once the producer path adds
+  // data-hf-authored-id to the host element. See PR #965 "Proper fix (follow-up)".
   it("documents the divergence: producer path lacks data-hf-authored-id element", () => {
     const document = makeHostDocument("intro");
     const host = document.querySelector('[data-composition-src="intro.html"]')!;