Live TV (ATSC/HDHomeRun): CEA-608 captions dropped by hardware (VAAPI) transcodes; Apple TV player

[bmoon 0]

## Environment
- Caption stream metadata: `Codec: eia_608, IsTextSubtitleStream: true, SubtitleLocationType: VideoSideData`, index 100.

It looks like AVPlayer's caption parsing for in-band 608 in a **progressive MPEG-2 TS** decodes the initial buffer and never processes subsequent caption data (in-band 608 handling is well supported for HLS/H.264 SEI; the progressive-MPEG-2-TS case appears unhandled or stalled).

## Finding 3 — no extraction fallback for live TV

Every `live.m3u8` request in the session log (all clients, ~18k requests) negotiate `SubtitleMethod=VideoSideData&VideoSideDataSubs=100&ManifestSubtitles=vtt` — i.e. captions are only ever carried **in-band**, and the `ManifestSubtitles=vtt` request never yields a WebVTT rendition for live streams. The transcode commands carry `-sn`. Server user policies expose no burn-in option per user. So when the in-band carrier is lost (Finding 1) or the player can't decode it (Finding 2), there is no third path — **a 608 → WebVTT extraction option for live TV would resolve all of the above**.

## Workaround that works today

Set the Apple TV app's player to **MVP** (the mpv-based player) and DirectStream the source (Max quality). mpv decodes the EIA-608 captions in software from the raw TS and renders them live:

- Captions update continuously (verified minutes at a time)
- Zero server transcode — the MPEG-2 stream is played as-is
- Quirk: after starting a session, the caption track sometimes needs to be re-selected (CC off → CC1) before mpv begins decoding — first selection at stream start appears to bind before caption decoding attaches

Android-family apps also expose a player choice; the same mpv route should fix captions there.

## Requests (in priority order)

1. **A53/CEA-608 caption injection for hardware H.264 encoders** in the transcode pipeline (VAAPI/QSV/NVENC), or transparent caption carry-over where the encoder supports user data — closes the silent caption loss for all HW-transcoding clients.
2. **Live TV subtitle extraction (608/708 → WebVTT)** as a server fallback when captions can't ride in-band — would rescue every affected client/player combination, including AVPlayer.
3. Investigate **in-band 608 decode stall in the Apple TV native player** on DirectStream of live MPEG-2 TS (Finding 2).
4. (Minor) AMD VAAPI intermittent `Access unit too large / Encode failed: -28` on live HLS transcodes, which surfaces the otherwise-unreachable software fallback.

Happy to attach the full ffmpeg transcode logs and session log excerpts on request.