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/hyperframes-animation/adapters/gsap.md

@@ -27,7 +27,7 @@ HyperFrames controls GSAP through its `gsap` runtime adapter. Create a paused ti
 - The registry key must match the composition root's `data-composition-id`.
 - Bracket and dot syntax both register: `window.__timelines["main"] = tl` and `window.__timelines.main = tl` are equivalent (the linter recognizes both). Bracket form is required when the id isn't a valid identifier (e.g. contains `-`).
 - Do not call `tl.play()` for render-critical motion.
-- Do not build timelines inside async code, timers, or event handlers.
+- Building inside an async callback such as `document.fonts.ready` is supported and common. What breaks is **registering the key before the build finishes**: an empty timeline registered early is treated as ready and nested empty, so it renders blank (`lint`: `gsap_timeline_registered_before_async_build`, error). Assign `window.__timelines[id] = tl` at the end of the callback. Do not drive render-critical motion from timers or event handlers.
 - Keep loops finite. HyperFrames renders finite video durations.
 - **Render duration comes from `data-duration` on the composition root, not from GSAP timeline length.** Do not pad the timeline with empty tweens like `tl.set({}, {}, 283)` to "extend" it. (Some external docs show this trick; in HyperFrames it conflicts with the seek-driven duration model — set `data-duration` instead.)
 
@@ -70,10 +70,10 @@ HyperFrames is stricter than vanilla GSAP. Animate only:
 
 **Forbidden** (breaks the renderer or the clip lifecycle):
 
-- `display`, raw `visibility` — never duration-tween these directly. Use `autoAlpha` (opacity plus endpoint visibility) or a zero-duration timeline set at an explicit boundary, and only on a non-clip element or wrapper inside a clip. Never target `.clip` itself.
+- `display`, raw `visibility` **on a clip element**: never duration-tween these. HyperFrames owns a clip's visibility and `lint` rejects it. Use `autoAlpha` (opacity plus endpoint visibility) or a zero-duration timeline set at an explicit boundary. Animating a clip element's other visual properties is fine and the shipped catalog does it throughout; what is forbidden is taking over its visibility.
 - Anything driven by `Math.random()`, `Date.now()`, `performance.now()`, or event handlers — animation state must be deterministic from time alone.
 
-> **Note**: `docs/guides/gsap-animation.mdx` lists `width`/`height`/`visibility` in its "Supported Properties" — that list is too permissive for HyperFrames composition rules. This allowlist is the canonical one. See `hyperframes-core/references/determinism-rules.md` for the full deterministic-render contract.
+> **Note**: the list above is a **denylist**, not an allowlist. Properties outside it, including `width`, `height`, `filter`, `clipPath` and `strokeDashoffset`, are legitimate targets; prefer transforms and opacity where you have the choice, for performance rather than correctness. See `hyperframes-core/references/determinism-rules.md` for the full deterministic-render contract.
 
 ## References