feat(studio): flat inspector — Media group

[vanceingalls]

What

Fourth PR in a 6-PR stack for the flat inspector redesign (see #2120 for the foundation and full stack list). Adds the FlatToggle primitive and the flat Media group (source/copy row, cutout sub-block, volume/rate/media-start sliders, loop/muted/has-audio-track toggles, fit/position rows), wired into the accordion.

Stack: #2120 (Foundation+Text) → #2121 (Style) → #2122 (Layout+Motion) → #2123 (this) → #2124 (Grade) → #2125 (Pinning + multi-field Text).

Why

Media covers every video/audio/image-specific control (playback, cutout, fit) that doesn't belong in the more general Style/Layout groups.

How

• FlatToggle is a new shared primitive (used here for loop/muted/has-audio-track, and reused by the Grade group in the next PR).
• FlatMediaSection composes six rows/sub-blocks across playback and framing controls.
• Still behind STUDIO_FLAT_INSPECTOR_ENABLED (default off).

Test plan

• Every row has a real DOM-event test with exact assertions — including branching toggle logic (e.g. muted vs. has-audio-track's two-sided onSetHtmlAttribute calls), which required an explicit fix-round during task review to reach full coverage.
• Full monorepo suite green; oxlint/oxfmt/fallow clean.
• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit #2127 : 3 dependent PRs (#2128 , #2129 , #2134 )
• fix(studio): retire legacy Style and Grade sections from flat inspector #2126
• feat(studio): flat inspector — persisted pinning + multi-field Text #2125
• feat(studio): flat inspector — Grade (color grading) group #2124
• feat(studio): flat inspector — Media group #2123 👈 (View in Graphite)
• feat(studio): flat inspector — Layout + Motion groups #2122
• feat(studio): flat inspector — Style group #2121
• feat(studio): flat inspector foundation + Text group #2120
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[miga-heygen]

Reviewed as part of the 15-PR edit-panel redesign stack. Full stack review posted on #2120. No blockers on this PR. — Miga

[miguel-heygen]

Final current-head pass: 4b9c3cf. Reviewed the stack boundaries and current diff against the flat-inspector contracts; required checks have no failures/pending checks and no unresolved review threads remain. No new drift found; residual notes are non-blocking.

[miguel-heygen]

Superseding earlier approval: the latest max-review pass found confirmed correctness issues in this flat-inspector stack (identity/selector-index state, hide-all write races, timing inference, slider pointer/keyboard/reset semantics, and duplicate React keys). Hold merge and require fixes plus re-review on the current stack head.