Add native loading=lazy support for video elements

[adamsilverstein]

Summary

Closes #2400.

• Adds loading="lazy" as a progressive enhancement for <video> elements that are below the fold (intersectionRatio = 0), alongside the existing JavaScript IntersectionObserver-based lazy loading
• Removes any existing loading attribute from LCP videos and visible videos to prevent unintended lazy loading
• Keeps the JS fallback (od-lazy-video class, data-original-* attributes, lazy-load-video.js) since browser support for loading="lazy" on video is still rolling out

This leverages the new HTML spec support for loading="lazy" on video elements (whatwg/html#10376). In unsupported browsers, the attribute is simply ignored (zero cost). Audio tag support is deferred to a separate PR.

Test plan

• Verify existing snapshot tests pass with updated expected outputs
• New test: hidden video gets loading="lazy" alongside JS lazy loading
• New test: visible video with loading="lazy" has the attribute removed
• New test: LCP video with loading="lazy" has the attribute removed
• PHPStan passes at level 10

AI Usage

I used Claude code to write the code and tests for this PR which I reviewed. It should probably be manually tested as well, I have not done that yet. The PR is mostly test cases, the code change itself is minor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.36%. Comparing base (8d1741e) to head (39f797e).

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2450      +/-   ##
==========================================
+ Coverage   69.33%   69.36%   +0.02%     
==========================================
  Files          90       90              
  Lines        7749     7755       +6     
==========================================
+ Hits         5373     5379       +6     
  Misses       2376     2376              

Flag	Coverage Δ
multisite	69.36% <100.00%> (+0.02%)	⬆️
single	35.70% <0.00%> (-0.03%)	⬇️

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[github-actions]

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Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[adamsilverstein]

I verified this worked as expected on a test page in my local. Non LCP video tags get the lazy loading attribute:

<figure class="wp-block-video"><video class="od-lazy-video" data-od-added-class data-od-added-data-original-preload data-od-added-loading data-od-added-preload data-od-xpath="/HTML/BODY/DIV[@id=&apos;page&apos;]/*[3][self::DIV]/*[1][self::DIV]/*[1][self::MAIN]/*[1][self::ARTICLE]/*[1][self::DIV]/*[12][self::FIGURE]/*[1][self::VIDEO]" data-original-preload="default" loading="lazy" preload="none" height="904" style="aspect-ratio: 1392 / 904;" width="1392" controls src="https://wpdev.localhost/wp-content/uploads/2026/04/inner-button-turns-to-save.mp4"></video></figure>