fix(skills,lint): correct composition-contract claims the code contradicts (#3468)

The runtime absorbed a series of authoring mistakes over time and `runtime/init.ts`
says so in its own comments, but the skills kept teaching the old rules. Four of
them actively cost an agent a failing run: add `crossorigin` (lint rejects it
unconditionally), never build a timeline inside `async` (lint calls that the
documented contract), never `gsap.set` later-scene clips (two fixHints instruct
exactly that), and 12 copyable media snippets with no `id`, which render silent.

Corrected in every place each claim appeared, including `hyperframes-animation`,
three workflow scripts, the scaffolded project instructions, the CLI `docs`
command, and the public docs site: `data-track-index` is a Studio display lane
the render never reads, `class="clip"` is a layout convention rather than a
visibility requirement, timed elements may nest, the visibility window is
half-open, sub-composition host dimensions are backfilled, and the root-fill rule
applies only to the layered-composite path.

Behaviour changes, each backed by a render rather than by reading code:

- `timeline_registry_missing_init` deleted. The runtime creates the registry
  before any inline script; a composition without the guard line renders and
  animates correctly.
- `video_nested_in_timed_element` kept, message corrected. A rendered repro shows
  the nested-with-local-start case really does break, so the rule guards a real
  defect, but nothing is "FROZEN": the extractor ignores the wrapper's offset
  while visibility uses it, so the clip shows wrong frames and then vanishes.
- `mediaRenderIds` now stamps media whose source is a `<source>` child, closing a
  duplicate-id gap the old `[src]`-only selector left open.
- Stale messages fixed on `subcomposition_root_styled_by_class` and
  `deprecated_data_layer`.

`coreSkillContent.test.ts` pinned the literal sentence that made root
`data-start` look required, so it is narrowed to structure plus the regression it
genuinely catches.

Not covered, and flagged in the PR: the media global-vs-local start heuristic in
`runtime/init.ts` is the root cause behind the nested-video defect. Removing it
changes the meaning of existing compositions and needs its own deprecation.

skills/pr-to-video/scripts/assemble-index.mjs

@@ -12,7 +12,10 @@
 // document order. Transitions are NOT written here — the transitions injector
 // mutates this file afterward (data-start/duration/track-index + GSAP).
 //
-// Track lanes (same-track time-overlap is illegal — lint timeline_track_too_dense):
+// Track lanes. Same-track time-overlap is this workflow's own assembly convention,
+// not a framework rule: the render never reads data-track-index, and no lint rule
+// checks overlap (timeline_track_too_dense counts elements per lane for readability).
+// The convention exists because the frame injector below ping-pongs 0/1 for overlaps:
 //   1      frame sub-comp clips (sequential; the injector 0/1-ping-pongs for overlaps)
 //   2      captions sub-comp clip (full-duration overlay, on top of frames)
 //   10     per-frame voice <audio>
@@ -216,7 +219,7 @@ function guardFrame(html, label) {
     for (let i = 1; i < list.length; i++) {
       if (list[i].start < list[i - 1].end - EPS) {
         errors.push(
-          `${label}: clips on track ${track} overlap (one ends at ${r3(list[i - 1].end)}s, the next starts at ${r3(list[i].start)}s) — same-track time-overlap causes a render conflict. Put them on distinct data-track-index lanes or fix their windows.`,
+          `${label}: clips on track ${track} overlap (one ends at ${r3(list[i - 1].end)}s, the next starts at ${r3(list[i].start)}s). This workflow's injector assumes one clip per lane at a time. The render itself tolerates the overlap; put them on distinct data-track-index lanes or fix their windows.`,
         );
         break; // one report per track is enough
       }

skills/pr-to-video/scripts/transitions.mjs

@@ -326,7 +326,8 @@ function runVerify(argv) {
     a.start < b.start + b.duration - EPS && b.start < a.start + a.duration - EPS;
   const fail = [];
 
-  // (4) global no same-track overlap (the lint invariant).
+  // (4) global no same-track overlap. This is this workflow's own lane convention,
+  //     not a lint rule: nothing in the framework rejects same-track overlap.
   const all = [...clips.values()];
   for (let i = 0; i < all.length; i++)
     for (let j = i + 1; j < all.length; j++) {