refactor: establish HTML folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 17 of the Option 5 folder-feature stack.

• Previous: refactor: establish DOMNodes folder feature #326
• Next: the Media folder layer

Review this PR as the incremental diff against #326.

Summary

• add the target src/html folder on the unqualified internal HTML feature
• move 61 non-media HTML interface modules out of the legacy broad DOM folder
• add the supported WebAPI.HTML -> WebAPI.DOMNodes + WebAPI.File + HTML feature
• expose the existing HTMLUnknownElement interface publicly
• preserve every existing flat HTML module path and binding signature

Media boundary

DomHTMLMediaElement, HTMLAudioElement, HTMLTrackElement, and HTMLVideoElement remain for the next Media layer. Their APIs directly reference Remote Playback, Picture-in-Picture, WebVTT, media-error, time-range, and playback-quality types. Moving them here would create an HTML <-> Media cycle because Media already depends on HTML.

Temporary state

• the four media-specific HTML interfaces remain in the legacy folder for one layer and move with Media next
• HTML.res is received transitively from DOMNodes because Element and ShadowRoot require its serialization/shadow-root options
• final helper-type cleanup remains deferred

Review focus

• HTML folder membership and the explicit media boundary
• the WebAPI.HTML dependency closure
• public exposure of HTMLUnknownElement

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,DOMPlatform,Animation,CSSOM,Geometry,DOMNodes,File,HTML
• npm run build
• npm test
• npm run format:check
• git diff --check

[jderochervlk]

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