docs(website-to-hyperframes): add load-bearing GSAP authoring rules

[ibrews]

Summary

Adds a new Load-bearing rules for animation authoring subsection to skills/website-to-hyperframes/references/step-6-build.md, capturing five GSAP authoring rules the linter cannot catch but that silently ship broken output — compositions that pass lint and render elements invisible, unscrubbing, or frozen.

Evidence

Surfaced from two independent website-to-hyperframes builds on 2026-04-20:

• spatial-deck/promo/hyperframes-auto/ — single-page SPA source
• ~/harvardxr-auto/ — marketing-site source

Both lint-clean on the first try. Both rendered cleanly only because we injected these rules into the sub-agent prompt before dispatch. Without them, sub-agents produced lint-passing compositions with:

• Hero images that entered then vanished (stacked y-entrance + scale Ken Burns tweens on one element)
• Auras that looked right in the studio preview but were missing from the rendered MP4 (standalone gsap.to() that didn't scrub)
• Scene transitions where elements flashed visible before their own entrance (gsap.from() immediateRender interacting with .clip boundaries)

Rules added

1. No iframes for captured content — don't seek deterministically, cannot scrub.
2. Never stack two transform tweens on one element — the second tween's immediateRender resets the first, element ends up invisible. With before/after code showing two fixes (combine into one fromTo, or split across parent/child wrappers).
3. Prefer tl.fromTo() over tl.from() inside .clip scenes — from()'s default immediateRender: true writes state before the scene's data-start is active and misbehaves under non-linear seeking.
4. Ambient pulses must attach to the seekable tl, never bare gsap.to() — standalone tweens run on wallclock, don't scrub, absent from rendered output.
5. Generalize the existing caption hard-kill rule to every scene-boundary exit — any element whose visibility changes at a beat boundary needs the deterministic tl.set() kill after its fade, not just captions.

Rules already present in the file (repeat: -1, data-start/data-duration on template roots, caption hard-kill) are untouched — these five are net-new or are generalizations.

Related observations (not in this PR, flagging for maintainers)

Two adjacent findings from the same builds, worth filing separately if you want them tracked:

• Linter heuristic inconsistency. Identical hard-kill patterns at beat boundaries trigger overlapping_gsap_tweens on some builds but not others. Stabilizing this heuristic would remove a source of sub-agent confusion.
• step-1-capture.md thin output on canvas-driven / deck-style sources. Single-page apps that navigate via keyboard or internal state produce sections: 0, CTAs: 0, scroll-screenshots: 1. A --deck-mode flag that navigates via keyboard and captures per-state — or a documented pre-seed-screenshots workaround — would help.

Happy to open either as a separate issue if useful.

Test plan

• Render the diff locally and confirm markdown formatting is consistent with the rest of step-6-build.md (code fences, hyphen bullets, em-dashes).
• Optional: re-run the website-to-hyperframes skill against a test site with these rules present vs. absent — the "ambient pulse doesn't scrub" failure is the easiest to reproduce.