fix: anchor cold sabr seeks on audio

Author: Priveetee

src/main/kotlin/dev/typetype/server/services/SabrPlaybackSessionService.kt

@@ -166,8 +166,9 @@ internal class SabrPlaybackSessionService(private val sessionStore: SabrSessionS
         generation: Long,
         primeSession: Boolean = false,
     ): Unit {
-        val sequence = playbackStartSequence(videoFormat, playerTimeMs)
-        val request = SabrSegmentRequest.media(videoFormat, sequence)
+        val targetFormat = if (primeSession) audioFormat else videoFormat
+        val sequence = playbackStartSequence(targetFormat, playerTimeMs)
+        val request = SabrSegmentRequest.media(targetFormat, sequence)
         val startMs = session.streamState.getSegmentStartMs(request.format, request.sequenceNumber).coerceAtLeast(0L)
         setReaderPosition(request.format, startMs, generation)
         if (startMs < session.streamState.getMinBufferedEndMs()) {

src/test/kotlin/dev/typetype/server/services/SabrPlaybackSessionServiceTest.kt

@@ -46,6 +46,9 @@ class SabrPlaybackSessionServiceTest {
         assertEquals(88_168L, result.startTimeMs)
         assertFalse(result.ready)
         assertEquals(88_168L, holder.playerTimeMs())
+        val request = holder.consumeForwardSeek()
+        assertEquals(audio.itag, request?.format?.itag)
+        assertEquals(9, request?.sequenceNumber)
         coVerify(exactly = 0) { store.preflightPlayback(holder, 88_168L) }
         verify { store.startPump(holder) }
         verify { store.warmPlaybackAsync(holder) }