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/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"]')!;