linter: flag missing tl.set() hard-kill on scene-boundary exits

[ibrews]

Background

PR #364 added Rule 5 to step-6-build.md:

Every element whose visibility changes at a beat boundary needs a deterministic tl.set() kill after its exit tween — not just captions. Without it, non-linear seeking can land between the fade end and the next scene start, leaving the element partially visible or causing a black-frame gap.

Problem

In a/b testing of PR #364 (see review #4167523103), the sub-agent produced 0 tl.set() hard-kills even with the rule present in the prompt — in both runs. The rule is documented but documentation alone isn't a reliable enforcement mechanism for sub-agents.

Meanwhile, scans of existing Stripe compositions found 86 exit animations with only 1 tl.set() hard-kill, and black-frame gaps at every beat boundary in rendered snapshots. The bug is real and widespread.

Proposed linter rule

Flag any element that has an exit tween ending at or near a data-start boundary without a corresponding tl.set() clearing its opacity/visibility at that same position.

Heuristic:

• Find tl.to(el, { opacity: 0, ... }, t) where t + duration ≈ a scene boundary
• Check whether a tl.set(el, { opacity: 0 }) (or equivalent) exists at the same position
• If not → warn: missing hard-kill after exit tween on <selector> at t=<time>

Notes

• The existing caption hard-kill check is already a narrower version of this. This would generalize it to all scene-boundary exits.
• False positive risk: elements that are hidden by other means (e.g., parent clip, display: none on the scene root). May need to be scoped to elements inside .clip containers only.
• Related: linter heuristic inconsistency on overlapping_gsap_tweens (separate issue) may be masking some of these cases.