Feature Worthington jets video on homepage

[VatsalSy]

Summary

• Feature the new Self-similar Worthington jets preprint in the homepage hero
• Embed the full research video with the paper while limiting the hero treatment to the first 20 seconds
• Preserve reduced-motion behaviour and clean mobile anchor navigation

Changes

• Add the Worthington-jets YouTube slide and paper link to _data/hero.yml
• Support per-slide hero durations and optional YouTube end times
• Reset media when carousel slides reactivate and disable YouTube autoplay/loop for reduced motion
• Add the privacy-enhanced full-video embed and a stable paper anchor to the research entry
• Clear the fixed header when navigating directly to paper anchors

Testing

• npm test -- --runInBand (83 tests passed)
• npm run lint:css (passed)
• ./scripts/lint-check.sh (passed)
• bash scripts/validate-content-rules.sh (passed)
• python3 scripts/check-content-rules-trigger-parity.py (passed)
• ./scripts/build.sh (passed)
• Desktop and mobile browser checks: no horizontal overflow; hero stayed active through 7 seconds and advanced after 20 seconds; reduced motion disabled autoplay and looping; the mobile paper anchor cleared the fixed header
• git diff --check (passed)

[Copilot]

Pull request overview

This PR updates the site’s homepage hero and research page to feature the new “Self-similar Worthington jets” preprint, including a short hero treatment and a full privacy-enhanced video embed, while improving slide timing/anchor behavior.

Changes:

• Adds a new Worthington jets YouTube hero slide with configurable per-slide dwell time and optional end= timestamp.
• Updates the homepage hero rotator logic to support per-slide durations and to reset media when slides reactivate, with reduced-motion handling for autoplay/loop parameters.
• Adds a stable anchor + full video embed to the research entry and adjusts scroll offsets for direct anchor navigation under a fixed header.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
index.html	Adds per-slide data-duration, supports optional YouTube end times, and updates the hero carousel runtime logic (including reduced-motion behavior).
assets/css/research-v2.css	Adds scroll offset handling (scroll-margin-top) for research paper containers/headings to avoid fixed-header overlap on anchor navigation.
_research/index.md	Adds a stable anchor for the Worthington jets entry and embeds the full YouTube (nocookie) video with privacy-related attributes.
_data/hero.yml	Adds the Worthington jets hero slide data and documents new per-slide duration and youtube_end fields.

Comments suppressed due to low confidence (1)

index.html:336

• Inactive hero slides are only hidden via opacity: 0 (see assets/css/home.css), so their <video> and especially the new YouTube <iframe> will continue playing in the background. This can waste bandwidth/CPU (particularly on mobile) even when the slide isn’t visible. Consider pausing the outgoing <video> and blanking the outgoing <iframe> src when a slide deactivates, then restoring from a stored data-src when the slide becomes active again.

    function advance() {
      figures[idx].setAttribute('data-active', 'false');
      figures[idx].setAttribute('aria-hidden', 'true');
      figures[idx].setAttribute('inert', '');
      idx = (idx + 1) % figures.length;

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