[chubes4]

Summary

Fixes #3583: video grids no longer become invalid media-text blocks. Imports preserve source order and the 860px breakpoint; browser-measured heading and video geometry now matches the reduced source at 390, 768, and 1440px.

What changed

• Keep video panes in native group layouts with editable core/video attributes.
• Remove unsupported media-text video serialization carriers.
• Match Gutenberg’s default preload save shape and preserve percentage-fill video geometry.
• Bind video attachment IDs through the existing Media Library resolver.
• Add reduced-fixture coverage for import, attachment resolution, Gutenberg save validation, and browser geometry.
• Video/text imports now produce groups containing core/video instead of core/media-text. Image conversion remains unchanged.

Verification

• cd php-transformer && composer install --no-interaction --quiet && composer test:unit && composer test:canonical && composer lint passed on 95709f3. Base trunk@e2a8c8c unchanged.
• CI: pending

Cook run

• Gate 1: tree 913f1402dd67ec8f085e0a8104756f48c0882251, fingerprint c7721a3c3c2f3cc36145db624df0ce6ba8336181bef69c5164088664f95aa542
• Publication identity: ambient_gh

AI assistance

Tool: Homeboy (pi) · Model: gpt-6.1-sol (tier roadie/cook-heavy) · Used for: cook-heavy running in the Pi coding agent reproduced the issue, implemented the fix, added regression tests, and ran validation.

Closes #3583