Mobile playback: video and audio with resume, tracks, background audio, touch seek #27

Part of #14 (W3 — mobile v1). Files and SDK playback prerequisites are complete. Folds in the native-Android half of #8 (touch seek gestures). Core mobile playback completed with #158.

Outcome

Watching and listening on the go — the mobile product bar's core media job, via the platform-native player path.

Scope

• Video playback (Media3/ExoPlayer): HLS/MP4 per the SDK playback-source model (#88, #89), configured format (#109), subtitle tracks with autoselect and lifecycle retention (#94, #100), autoplay next (#110)
• Resume prompt and throttled start_from write-back (#158): 15-second playing samples, deduplicated lifecycle snapshots, bounded writer, session and confirmed-setting checks. Start over starts locally at zero; completion keeps the final positive position.
• Audio track selection and playback speed for audio and video — completed under #146 in #154; retained selections and the 0.75 / 1 / 1.25 / 1.5 / 2× ladder.
• Touch controls per Add touch seek gestures and controls to Android playback #8: double-tap left/right accumulating 10s skips with pending-total feedback, clamped at bounds, no conflict with single-tap/captions/system nav, plus an accessible non-gesture alternative
• Audio playback with background audio + MediaSession — completed under #146 in #150, #151, and #152; live-session reopening in #155.
• Conversion interstitial for need_convert files (queued/percent), Check again, and an explicit unsupported-type state — source resolution in #88, player UI in #89. Current UI and regression coverage verify percentage, refresh/Back, and unsupported without retry.
• Typed, recoverable playback errors (#88, #89): media-credential refresh, network/generic recovery, retained position, and authoritative session rejection. Sentry playback breadcrumbs remain conditional on telemetry and are owned by #31; no telemetry delivery is claimed here.

Non-goals (v1)

PiP and libVLC/original-file playback remain post-v1 under #52. Chromecast is a mobile-v1 child in #36 and follows this core player.

Acceptance criteria

• DoD: canonical local gate and final-head CI green; API 37 recordings and screenshots attached across the playback PRs (evidence below).

Evidence

• Resume and Start over for video/audio, live position read-back, background reporting and notification Pause. Owned API fixtures were reset, deleted and verified absent; account settings and the authenticated installation were preserved.
• Decoded HLS and rendered subtitles, touch-seek recordings, speed/audio-track selection and retention, and immersive video with direct Audio/Speed/Captions controls. Selection evidence includes screenshots and controlled local-media device assertions; it is not a live API or full Activity-recreation claim.
• Background audio, lock-screen/headset controls and notification reopening.
• Conversion, unsupported and typed recovery completion is based on the shipped implementation and passing regression coverage. This closeout does not claim a new live queued-conversion or unsupported-file recording.

This closes the core player scope. Downloads, share/open-in, Chromecast, telemetry/release and post-v1 player features remain with their respective issues.