fix: add regression fixtures with golden baselines + address review

- Create sub-comp-t0 and sub-comp-id-selector as proper regression tests
  under packages/producer/tests/ with golden MP4 baselines
- Add both to shard-7 in regression.yml
- Add clarifying comment on activateNestedChildTimelines scope
- Confirm test fixture network safety in comment

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

@@ -2,6 +2,8 @@ import { describe, expect, it } from "vitest";
 import { parseHTML } from "linkedom";
 import { inlineSubCompositions } from "./inlineSubCompositions";
 
+// Fixtures reference GSAP CDN but are never loaded in a real browser — resolveHtml is mocked.
+
 /**
  * Minimal sub-composition HTML that uses `#intro` as its CSS and GSAP scope.
  * This is the pattern that breaks when the producer path strips the inner root.