fix(android): stop background audio rewinding to where it started

A video handed off to background audio (UR-040) streams a live mp3 transcode
over plain HTTP. That response is chunked, so there is no Content-Length, and a
live encode carries no Xing header, so the extractor establishes no duration —
on device every position tick reads "<position> / 0.0".

ProgressiveMediaPeriod.configureRetry resumes a failed load in place only when
the content length is known or the seek map has a duration. With neither it
assumes the source is live, sets pendingDeferredRetry, and when the sample
queues next run dry resets them and re-requests the URL from offset 0. Our URL
carries StartTimeTicks = the handoff point, so "offset 0" is where audio-only
mode began: a transient load error armed a retry that fired minutes later, when
the buffer finally drained, and playback resumed at the handoff point and ran
on from there. A successful retry raises no error and ends nothing, so neither
arm of DR-129 was consulted and no discontinuity handler existed — the only
trace was a position that went backwards, which is why it read as random, and
why the two earlier fixes for the same symptom (DR-129's phantom end, DR-159's
relative-timeline leak) left it standing.

A retry that can only restart the stream is worth less than no retry at all.
player_retry_restarts_stream marks a Remote audio-only video item,
loadWithMetadata carries the answer to Kotlin, and the pure StreamRetryDecision
holds it for a DefaultLoadErrorHandlingPolicy that returns C.TIME_UNSET —
making onLoadError answer DONT_RETRY_FATAL before it reaches configureRetry.
The rewind becomes a recoverable error, which recoverable_error_resume already
answers by re-opening at the position playback reached, StartTimeTicks
rewritten so the selected audio track survives. Every other source keeps the
player's retry: a static file and an HLS playlist declare their timeline and
are resumed where the load stopped. onPositionDiscontinuity is added for its
log line alone, loud for DISCONTINUITY_REASON_INTERNAL, which is the rewind's
own signature.

Verified on device (FP5), same procedure both runs — handoff, 60s to fill the
buffer, a 45s radio outage:

  before  13:54:52 BUFFERING, then "Media ready! Duration: -9.22e15"
          (C.TIME_UNSET) and position 1165.4s -> 840.349s, exactly the handoff
          base, 3.5 minutes after the outage with nothing logged between
  after   14:05:08 "declining the player's retry", playback undisturbed off the
          buffer for 69s (a fatal load error is only raised when the renderer
          next needs data), then ERROR_CODE_IO_NETWORK_CONNECTION_FAILED ->
          re-opening at 785.6s -> READY, and no rewind in the following 7 min

Kotlin tests run with ./gradlew :app:testUniversalDebugUnitTest.

TRACES: UR-040, UR-004 | DR-203 | UT-200

src-tauri/android/src/main/java/com/dtourolle/jellytau/player/JellyTauPlayer.kt

@@ -20,6 +20,9 @@ import androidx.media3.common.PlaybackException
 import androidx.media3.common.Player
 import androidx.media3.common.util.UnstableApi
 import androidx.media3.exoplayer.ExoPlayer
+import androidx.media3.exoplayer.source.DefaultMediaSourceFactory
+import androidx.media3.exoplayer.upstream.DefaultLoadErrorHandlingPolicy
+import androidx.media3.exoplayer.upstream.LoadErrorHandlingPolicy
 import kotlinx.coroutines.*
 
 /**
@@ -256,6 +259,45 @@ class JellyTauPlayer(private val appContext: Context) {
      *  (and leak) a focus request we already own. */
     private var hasAudioFocus = false
 
+    /**
+     * Whether the stream that is loaded may be retried by the player itself.
+     *
+     * Set from Rust on every load; see [StreamRetryDecision] for why the
+     * background-audio handoff transcode must answer no. (DR-203)
+     */
+    private val streamRetry = StreamRetryDecision()
+
+    /**
+     * The default retry behaviour, except that a stream the player could only
+     * restart is not retried at all.
+     *
+     * `C.TIME_UNSET` makes `ProgressiveMediaPeriod.onLoadError` return
+     * `DONT_RETRY_FATAL` *before* it reaches `configureRetry`, which is the
+     * method that would otherwise reset the sample queues and re-request the URL
+     * from offset 0. The error then surfaces through [onPlayerError] as
+     * recoverable, and Rust re-opens the stream at the position playback
+     * actually reached (DR-129).
+     *
+     * TRACES: UR-040, UR-004 | DR-203
+     */
+    private val loadErrorHandlingPolicy: LoadErrorHandlingPolicy =
+        object : DefaultLoadErrorHandlingPolicy() {
+            override fun getRetryDelayMsFor(
+                loadErrorInfo: LoadErrorHandlingPolicy.LoadErrorInfo
+            ): Long {
+                if (!streamRetry.playerMayRetry) {
+                    android.util.Log.w(
+                        "JellyTauPlayer",
+                        "Load error on a stream that cannot be resumed in place — " +
+                            "declining the player's retry so the backend can re-open it: " +
+                            "${loadErrorInfo.exception}"
+                    )
+                    return C.TIME_UNSET
+                }
+                return super.getRetryDelayMsFor(loadErrorInfo)
+            }
+        }
+
     init {
         // Configure audio attributes for music playback with audio focus handling
         val audioAttributes = AudioAttributes.Builder()
@@ -273,6 +315,13 @@ class JellyTauPlayer(private val appContext: Context) {
         //
         // TRACES: UR-004, UR-006 | IR-008
         exoPlayer = ExoPlayer.Builder(appContext)
+            // Decline the player's own load-error retry for a stream it could
+            // only restart (DR-203). Every other source keeps the default
+            // behaviour, which resumes the failed load where it stopped.
+            .setMediaSourceFactory(
+                DefaultMediaSourceFactory(appContext)
+                    .setLoadErrorHandlingPolicy(loadErrorHandlingPolicy)
+            )
             .setAudioAttributes(audioAttributes, /* handleAudioFocus= */ true)
             // Pause when the audio output is removed (wired headphones unplugged or
             // Bluetooth device disconnected). ExoPlayer listens for the system
@@ -354,6 +403,33 @@ class JellyTauPlayer(private val appContext: Context) {
                 updatePlaybackServiceNotification(isPlaying)
             }
 
+            /**
+             * A jump in the timeline nobody asked for.
+             *
+             * Logged rather than acted on: with the load-error retry declined for
+             * streams that can only be restarted (DR-203), a backwards
+             * `DISCONTINUITY_REASON_INTERNAL` here means the player rewound one
+             * anyway, and this line is what would show it.
+             */
+            override fun onPositionDiscontinuity(
+                oldPosition: Player.PositionInfo,
+                newPosition: Player.PositionInfo,
+                reason: Int
+            ) {
+                val message = "▶ Position discontinuity: ${oldPosition.positionMs}ms -> " +
+                    "${newPosition.positionMs}ms (reason=$reason)"
+                if (reason == Player.DISCONTINUITY_REASON_INTERNAL) {
+                    // The player moved the timeline of its own accord — the
+                    // signature of the DR-203 rewind. Loud, because with the
+                    // retry declined it should no longer be reachable.
+                    android.util.Log.w("JellyTauPlayer", "$message — player-initiated")
+                } else if (newPosition.positionMs < oldPosition.positionMs - 1000) {
+                    // Backwards, but asked for: a seek, or the re-prepare a
+                    // stream resume does (reason REMOVE). Normal, so quiet.
+                    android.util.Log.d("JellyTauPlayer", message)
+                }
+            }
+
             override fun onPlayerError(error: PlaybackException) {
                 android.util.Log.e("JellyTauPlayer", "▶▶▶ PLAYER ERROR: ${error.errorCodeName}", error)
                 android.util.Log.e("JellyTauPlayer", "  Error code: ${error.errorCode}")
@@ -845,11 +921,16 @@ class JellyTauPlayer(private val appContext: Context) {
         artworkUrl: String?,
         durationMs: Long,
         mediaType: String = "audio",
-        subtitlesJson: String = "[]"
+        subtitlesJson: String = "[]",
+        nonResumableStream: Boolean = false
     ) {
         mainHandler.post {
             currentMediaId = mediaId
             endedNotified = false
+            // Who owns recovery for this stream, decided in Rust (DR-203). Set
+            // before prepare(), since the first load error can arrive as soon as
+            // the player starts reading.
+            streamRetry.onLoad(nonResumableStream)
 
             // Store metadata for notification updates
             currentTitle = title

src-tauri/android/src/main/java/com/dtourolle/jellytau/player/StreamRetryDecision.kt

@@ -0,0 +1,50 @@
+package com.dtourolle.jellytau.player
+
+/**
+ * Whether the *player* is allowed to retry a failed load of what is currently
+ * loaded, or whether recovery belongs to the backend instead.
+ *
+ * Pure state, deliberately free of any media3 or Android type so the decision is
+ * unit-testable off-device — the same shape as `ScreenWakeState` (DR-202).
+ *
+ * ExoPlayer resumes a failed load in place only when it knows where "in place"
+ * is: `ProgressiveMediaPeriod.configureRetry` keeps the load position when the
+ * content length is known *or* the extractor produced a seek map with a
+ * duration, and otherwise assumes the source is live — it resets every sample
+ * queue and re-requests the URL from offset 0.
+ *
+ * The background-audio handoff transcode (UR-040) satisfies neither condition:
+ * `/Audio/{id}/universal?Container=mp3&TranscodingProtocol=http` is chunked, so
+ * there is no `Content-Length`, and a live mp3 encode carries no `Xing` header,
+ * so the duration is unset — visible in logcat as every position tick reading
+ * `<position> / 0.0`. Its URL carries `StartTimeTicks` = the handoff point, so a
+ * restart from offset 0 drops playback back to where audio-only mode began and
+ * carries on from there, and because that is a successful *retry* rather than a
+ * failure, no error and no `STATE_ENDED` is ever reported: the app cannot see it
+ * happen. That is the bug this exists to prevent (DR-203).
+ *
+ * Rust decides which streams those are and says so on every load; this only
+ * remembers the answer for the load-error policy to read. Refusing the retry
+ * turns the silent rewind into a recoverable error, which the backend answers by
+ * re-opening the stream at the position playback actually reached (DR-129).
+ *
+ * TRACES: UR-040, UR-004 | DR-203 | UT-200
+ */
+class StreamRetryDecision {
+    @Volatile
+    private var nonResumableStream = false
+
+    /**
+     * Record what is being loaded.
+     *
+     * @param nonResumable whether re-requesting this stream would restart it
+     *   rather than continue it — `player_retry_restarts_stream` in Rust.
+     */
+    fun onLoad(nonResumable: Boolean) {
+        nonResumableStream = nonResumable
+    }
+
+    /** True while the player may handle a load error by retrying it itself. */
+    val playerMayRetry: Boolean
+        get() = !nonResumableStream
+}

src-tauri/android/src/test/java/com/dtourolle/jellytau/player/StreamRetryDecisionTest.kt

@@ -0,0 +1,47 @@
+package com.dtourolle.jellytau.player
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * Who owns recovery for the stream that is loaded.
+ *
+ * TRACES: UR-040, UR-004 | DR-203 | UT-200
+ */
+class StreamRetryDecisionTest {
+
+    /** Nothing loaded yet is an ordinary stream: the player retries as it always has. */
+    @Test
+    fun `starts allowing the player to retry`() {
+        assertTrue(StreamRetryDecision().playerMayRetry)
+    }
+
+    /**
+     * The reported bug: the length-less handoff transcode can only be "retried"
+     * from its beginning, which replays the episode from the handoff point
+     * without reporting anything. The player must not be allowed to try.
+     */
+    @Test
+    fun `a non-resumable stream refuses the player its retry`() {
+        val decision = StreamRetryDecision()
+        decision.onLoad(nonResumable = true)
+        assertFalse(decision.playerMayRetry)
+    }
+
+    @Test
+    fun `an ordinary stream keeps the player retry`() {
+        val decision = StreamRetryDecision()
+        decision.onLoad(nonResumable = false)
+        assertTrue(decision.playerMayRetry)
+    }
+
+    /** The next load decides for itself — the handoff must not outlive its item. */
+    @Test
+    fun `loading an ordinary stream after a handoff restores the retry`() {
+        val decision = StreamRetryDecision()
+        decision.onLoad(nonResumable = true)
+        decision.onLoad(nonResumable = false)
+        assertTrue(decision.playerMayRetry)
+    }
+}