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-core/references/variables-and-media.md

@@ -95,8 +95,10 @@ Video elements must be muted and inline. Audio must be a separate `<audio>` elem
 - **Do not** call `video.play()`, `audio.play()`, pause, or seek in composition code. HyperFrames owns playback.
 - **Do not** drive host-root media from a sub-comp timeline: a sub-comp timeline cannot reach elements outside its subtree, so it has no effect. Drive host-root media from the main timeline at global time (or keep the media inside the sub-comp whose timeline animates it).
 - **Do not** animate timed media element dimensions; animate a non-timed wrapper instead.
-- **Do not** nest video inside a timed wrapper. Put timing on the media element or keep the wrapper untimed.
-- Add `crossorigin="anonymous"` for external media that needs canvas capture or pixel inspection.
+- **Do not** nest video inside a timed wrapper. `lint` rejects a `<video data-start>` whose ancestor also carries `data-start` (`video_nested_in_timed_element`, error), and the failure is real: the frame extractor resolves the video's start from its own `data-start` without the wrapper's offset, while visibility uses the wrapper's window. The clip then shows the wrong source frames and disappears partway through its slot. Put the timing on the wrapper **or** on the media element, never both.
+- **Sub-compositions are exempt and work.** A `<video>`/`<audio>` inside a sub-composition renders identically to one at the host root, because a composition host propagates its offset. Only plain timed wrappers (a `<section data-start>` around a `<video data-start>`) break.
+- **Never** add `crossorigin` to `<video>`/`<audio>`. `lint` rejects it unconditionally (`media_crossorigin_breaks_preview`, error) because a media host without `Access-Control-Allow-Origin` then fails silently in preview while renders still work, hiding the bug. There is no suppression, so this holds even for the canvas/WebGL/WebAudio readback case.
+- **Every `<audio>` needs an `id`.** The mixer selects `audio[id][src]`, so an id-less `<audio>` is never mixed and the render is **silent**. `lint` catches it as `media_missing_id`.
 - Audio always lives on a separate `<audio>` element — even if its source file is the same as a `<video>`. The `<video>` is muted; the `<audio>` carries sound.
 - For volume fades/ducking, animate `volume` on the timeline (`tl.to("#bgm", { volume: 0, duration: 1 }, "outro")`) rather than swapping `data-volume`. The runtime probes the timeline's volume keyframes and applies them identically in preview and render; `data-volume` is the static baseline for elements no tween touches. A tween's values REPLACE that baseline rather than scaling it, so on a clip whose gain is not `1` you scale the tween's targets instead (`{ volume: 1.95 }`, not `{ volume: 1 }`) — `lint` warns with `audio_volume_tween_overrides_gain` when the two disagree.