refactor: establish Media folder feature - #328

Tracking issue: #342

Stack context

Part 18 of the Option 5 folder-feature stack.

• Previous: refactor: establish HTML folder feature #327
• Next: the WebAudio folder layer

Review this PR as the incremental diff against #327.

Summary

• add the target src/media folder on the unqualified internal Media feature
• add the supported WebAPI.Media -> WebAPI.HTML + Media feature
• consolidate Encrypted Media Extensions, Media Capabilities, Media Capture and Streams, Media Session, Picture-in-Picture, Remote Playback, Web Speech, and WebVTT into the Media folder
• move the remaining media-specific HTML interfaces from the legacy DOM folder
• move BaseEncryptedMediaExtensions out of the foundational core
• preserve existing flat module paths and expose MediaError, VideoPlaybackQuality, and the consolidated media interfaces through WebAPI.Media

Temporary state

• Navigator remains in the legacy DOM folder and references Media-owned types; its broad cross-domain surface is resolved in the later Navigator integration PR
• Document remains in the legacy DOM folder even though it contains Picture-in-Picture operations; it moves with the later Window/document integration layer
• the legacy broad WebAPI.DOM source entry remains until its remaining interfaces have been assigned
• final feature-graph guardrails and helper cleanup remain deferred until the stack is complete

Review focus

• membership of the consolidated Media domain
• the WebAPI.Media dependency closure
• the boundary between general HTML interfaces and media-specific HTML interfaces
• newly public flat media interface modules

Verification

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