Feature: Hide Unavailable Media Radio Groups

## Context Media-backed radio groups should disappear when they have no meaningful choices. Audio tracks, captions, playback rates, and quality options need one shared hidden-state contract instead of framework-specific checks against raw availability. ## Acceptance criteria - [x] Core radio-group state derives a shared `hidden` value from availability. - [x] HTML exposes `data-hidden` and the native `hidden` attribute. - [x] React option hooks expose the same hidden state. - [x] State transitions are covered across core, HTML, and React. - [x] Documentation describes the shared contract. ## Resolution Resolved by #2069.