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/full-screen-motion.md

@@ -9,6 +9,19 @@ Stacking opaque scene divs means every scene change has to repaint the entire fr
 ## Pattern
 
 ```html
+<style>
+  /* The runtime auto-positions root children that carry data-start. The shared
+     background deliberately has none, so it gets NO automatic layout and must
+     size itself, or #bg is 0px tall and the tween paints nothing. */
+  #bg.full-bleed {
+    position: absolute;
+    inset: 0;
+  }
+  .clip.transparent {
+    background: transparent;
+  }
+</style>
+
 <div id="root" data-composition-id="main" data-width="1920" data-height="1080" data-duration="20">
   <!-- Shared background — NOT a clip. Always visible. Driven by the timeline. -->
   <div id="bg" class="full-bleed"></div>
@@ -51,7 +64,8 @@ Stacking opaque scene divs means every scene change has to repaint the entire fr
 
 ## Rules
 
-- **The background is not a clip.** No `data-start` / `data-duration` / `data-track-index`. It exists for the whole composition.
+- **The background is not a clip.** No `data-start` / `data-duration`. It exists for the whole composition.
+- **Because it is not a clip, it gets no automatic layout.** The runtime only positions and sizes root children that carry `data-start`. An untimed background must set its own `position: absolute; inset: 0`, or it collapses to zero height and nothing you animate on it is visible. This is the most common way this pattern is copied wrong.
 - **Content scenes have transparent backgrounds.** Whatever you put in the shared `#bg` shows through.
 - **Drive global state from the shared layer.** Hue shifts, vignettes, grain, film-look filters — animate them once on the shared layer, not per-scene.
 - **Do not animate visibility on `.clip` elements.** HyperFrames already shows/hides clips based on `data-start` and `data-duration`. Animating `display` / `visibility` on the clip itself races with the framework's own show/hide. Animate a _child wrapper_ inside the clip instead.