fix(player): stop the server burning subtitles into the picture

A transcoded episode stalled every few seconds and seeking took five to
nine seconds to produce a frame. Neither was a seek bug: both seeks in the
capture landed correctly. The stream itself could not keep up.

The episode was HEVC video, E-AC-3 audio, and a PGSSUB subtitle track.
Only the audio needed transcoding — the device profile supports HEVC and
the server would have remuxed the video untouched. But the PlaybackInfo
request omitted SubtitleStreamIndex, and omitting it does not mean "no
subtitles": the server then honours the source's default/forced flag and
picks a track itself. It picked the PGS one. PGS is a bitmap, and the
profile advertised only srt/vtt as External, so it could not go out as a
sidecar — leaving SubtitleMethod=Encode, burn-in.

Burn-in is a video cost, not a subtitle cost. Compositing rules out
remuxing, so the whole HEVC stream was re-encoded to h264 frame by frame.
The server could not sustain that in real time: the buffer never grew past
one segment and playback ran waiting -> HLS error -> canplay -> three
seconds of picture, indefinitely, while each seek restarted the encoder
from scratch. TranscodeReasons named it — SubtitleCodecNotSupported — but
nothing in the log connected that to the stall, so the diagnostic now says
which track it is declining and why.

Ask for SubtitleStreamIndex=-1 explicitly, and advertise every text format
we can render (srt/subrip/ass/ssa/vtt) as External so a subtitle can only
ever arrive as a sidecar. Nothing is lost: the app already fetches subtitle
tracks itself and draws them over the video (UR-020), so the server's
composited copy was always redundant. Image-based tracks are consequently
not offered, which is honest rather than a regression — the renderer cannot
composite a bitmap, and the previous behaviour paid for them by making the
stream unwatchable.

The policy lives beside the other device-profile rules in Rust, where it is
testable without a device.

TRACES: UR-020, UR-004 | DR-176 | UT-168

src-tauri/src/repository/device_profile.rs

@@ -61,6 +61,67 @@ const WEBVIEW_AUDIO_CODECS: &[&str] = &["aac", "mp3", "opus", "vorbis", "flac"];
 /// it rather than give up.
 const FALLBACK_AUDIO_CODEC: &str = "aac";
 
+/// Jellyfin's sentinel for "negotiate no subtitle stream at all".
+///
+/// Omitting `SubtitleStreamIndex` does **not** mean this: the server then applies
+/// the source's default/forced flags and picks a track itself. See
+/// [`playback_subtitle_stream_index`] for why that is never what we want.
+pub const NO_SUBTITLE_STREAM: i32 = -1;
+
+/// Subtitle formats we can render ourselves, delivered as an external sidecar
+/// track rather than painted into the video.
+///
+/// Every entry here is *text*. Image-based subtitles (PGS, DVD, DVB) are
+/// deliberately absent: they are bitmaps, so the only way a server can show them
+/// on a client that cannot composite them is to burn them into the picture.
+const EXTERNAL_SUBTITLE_FORMATS: &[&str] = &["srt", "subrip", "ass", "ssa", "vtt"];
+
+/// The `SubtitleProfile` entries to advertise, as `(format, method)`.
+///
+/// All `External`: the app fetches subtitle tracks itself and renders them over
+/// the video (UR-020), so it never needs the server to composite them.
+///
+/// TRACES: UR-020 | DR-176 | UT-168
+pub fn subtitle_profiles() -> Vec<(&'static str, &'static str)> {
+    EXTERNAL_SUBTITLE_FORMATS
+        .iter()
+        .map(|format| (*format, "External"))
+        .collect()
+}
+
+/// Whether asking the server to serve this subtitle codec forces it to burn the
+/// subtitle into the picture.
+///
+/// Burn-in is not a subtitle cost — it is a *video* cost. It rules out remuxing
+/// the video stream, so a source we would otherwise have passed through untouched
+/// gets fully re-encoded frame by frame.
+///
+/// TRACES: UR-020 | DR-176 | UT-168
+pub fn subtitle_forces_burn_in(codec: &str) -> bool {
+    !EXTERNAL_SUBTITLE_FORMATS
+        .iter()
+        .any(|format| format.eq_ignore_ascii_case(codec.trim()))
+}
+
+/// The `SubtitleStreamIndex` to negotiate with: always "none".
+///
+/// The reported bug: a source with an E-AC-3 track and a **PGSSUB** default
+/// subtitle track. Sending no index let the server honour that default, and since
+/// PGS cannot go out as a sidecar it chose `SubtitleMethod=Encode` — burn-in.
+/// That turned an audio-only transcode (the HEVC video was directly supported)
+/// into a full HEVC→h264 re-encode, which the server could not sustain in real
+/// time: the buffer never grew beyond one segment and playback stalled every few
+/// seconds, taking seeking down with it.
+///
+/// Asking for no subtitle stream costs nothing, because the app never wanted the
+/// server's composited version — it fetches the text tracks separately and
+/// renders them itself (UR-020).
+///
+/// TRACES: UR-020, UR-004 | DR-176 | UT-168
+pub fn playback_subtitle_stream_index() -> i32 {
+    NO_SUBTITLE_STREAM
+}
+
 /// Narrow a detected audio-codec list to what the renderer that will actually
 /// play the **video** can decode.
 ///
@@ -162,6 +223,43 @@ pub fn served_audio_codec<'a>(streams: &[(Option<&'a str>, bool)]) -> Option<&'a
 mod tests {
     use super::*;
 
+    /// The reported bug, at the level it was decided: a source whose default
+    /// subtitle track is PGSSUB must not drag the video into a re-encode.
+    ///
+    /// TRACES: UR-020 | DR-176 | UT-168
+    #[test]
+    fn no_subtitle_stream_is_negotiated_so_the_server_never_burns_one_in() {
+        assert_eq!(playback_subtitle_stream_index(), NO_SUBTITLE_STREAM);
+        // Not `None`/omitted: that is what let the server pick the PGS track.
+        assert_eq!(playback_subtitle_stream_index(), -1);
+    }
+
+    /// TRACES: UR-020 | DR-176 | UT-168
+    #[test]
+    fn text_subtitles_are_advertised_as_external_sidecars() {
+        let profiles = subtitle_profiles();
+        for format in ["srt", "subrip", "ass", "ssa", "vtt"] {
+            let entry = profiles.iter().find(|(f, _)| *f == format);
+            assert!(
+                entry.is_some(),
+                "{format} must be advertised or the server burns it into the picture"
+            );
+            assert_eq!(entry.unwrap().1, "External");
+        }
+    }
+
+    /// TRACES: UR-020 | DR-176 | UT-168
+    #[test]
+    fn text_subtitles_never_force_burn_in_but_image_ones_do() {
+        // Text: deliverable as a sidecar, so the video can still be remuxed.
+        assert!(!subtitle_forces_burn_in("subrip"));
+        assert!(!subtitle_forces_burn_in("ASS"));
+        assert!(!subtitle_forces_burn_in("ssa"));
+        // Image formats are bitmaps — the server can only composite them.
+        assert!(subtitle_forces_burn_in("PGSSUB"));
+        assert!(subtitle_forces_burn_in("dvdsub"));
+    }
+
     #[test]
     fn an_undecodable_default_track_forces_a_transcode() {
         // The reported bug: one E-AC-3 track, which the webview cannot decode.