fix(lint): remove root_composition_missing_data_duration (#490)

* fix(lint): remove root_composition_missing_data_duration

Lint cannot statically observe the runtime's true Infinity-emission
condition: it requires a finite GSAP timeline duration AND a finite
media/sub-comp window AND timeline > floor + 1, none of which are
visible to the static linter. The looping shapes that drive the
condition are already covered by `gsap_infinite_repeat` and
`gsap_repeat_ceil_overshoot` (both from #243), which point at the
real authoring mistake — flagging the missing duration separately
was a noisy proxy for the same signal.

Per #490 review discussion, deprecate the static rule and let those
two GSAP rules carry the pre-render coverage. If perfect precision
on `durationInFrames = Infinity` is needed, that belongs on the
runtime/render path where `shouldEmitNonDeterministicInf` is
actually known.

Add regression tests pinning the removal: a docs-compliant root
without `data-duration` no longer warns, and the canonical
loop-inflated shape now surfaces only via `gsap_infinite_repeat`
instead of two duplicate findings.

* docs(skills): update step-6 build checklist after rule removal

Drops the `root_composition_missing_data_duration` reference now that
the rule is gone. Keeps the authoring recommendation (and explains the
runtime Infinity case) but points authors at the GSAP rules that
actually carry the lint signal: `gsap_infinite_repeat` and
`gsap_repeat_ceil_overshoot`.

skills/website-to-hyperframes/references/step-6-build.md

@@ -114,7 +114,7 @@ After building the composition, check WITH ACTUAL CODE:
 - [ ] No full-screen dark linear gradients (H.264 creates visible banding — use solid + localized radial glows)
 - [ ] Timeline registered: `window.__timelines["comp-id"] = tl`
 - [ ] Colors match DESIGN.md exactly (paste the HEX value, don't approximate)
-- [ ] **Every `<template>` root element** — not just `index.html`, but every sub-composition's root — has `data-start="0"` **and** `data-duration="<beat_seconds>"`. The linter warns `root_composition_missing_data_start` / `root_composition_missing_data_duration` when missing; without `data-duration` the runtime may infer `Infinity` on repeating animations and stall playback.
+- [ ] **Every `<template>` root element** — not just `index.html`, but every sub-composition's root — has `data-start="0"`. The linter warns `root_composition_missing_data_start` when missing. Authoring `data-duration="<beat_seconds>"` on the root is also recommended for compositions whose GSAP timeline uses repeating animations (`repeat: -1` or large `repeat: N`); without it the runtime may infer `Infinity` and stall playback. The linter flags those repeating shapes directly via `gsap_infinite_repeat` and `gsap_repeat_ceil_overshoot`.
 - [ ] **Caption exits have a hard kill.** If you animate captions out with `tl.to(groupEl, { opacity: 0 }, group.end)`, follow it with `tl.set(groupEl, { opacity: 0, visibility: "hidden" }, group.end)` as a deterministic kill — per-word karaoke tweens can override the exit tween and leave captions stuck on screen. Linter: `caption_exit_missing_hard_kill`.
 - [ ] **No duplicate media nodes.** If the same image/video source is referenced twice with identical `data-start` + `data-duration`, the compiler discovers it twice and can double-render. Dedupe by using a single `<img>` with appropriate z-layering, or stagger the `data-start` values. Linter: `duplicate_media_discovery_risk`.