audio: send the AAC codec config to AAC-only sinks

A GalReceiver 2014 head unit, whose media sink takes only raw AAC-LC,
reset the connection on the first audio frame. Raw AAC-LC has no header
of its own, so send the AudioSpecificConfig right after Start, as video
does with SPS/PPS. PCM and ADTS sinks are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

gearslip/src/main/java/app/seb3thehacker/gearslip/audio/AacEncoder.kt

@@ -87,8 +87,17 @@ internal class AacEncoder(private val sampleRate: Int, private val channels: Int
         runCatching { codec.release() }
     }
 
-    private companion object {
-        const val INPUT_WAIT_US = 10_000L
-        val FREQUENCIES = listOf(96000, 88200, 64000, 48000, 44100, 32000, 24000, 22050, 16000, 12000, 11025, 8000, 7350)
+    companion object {
+        private const val INPUT_WAIT_US = 10_000L
+        private val FREQUENCIES = listOf(96000, 88200, 64000, 48000, 44100, 32000, 24000, 22050, 16000, 12000, 11025, 8000, 7350)
+
+        /** The two-byte AudioSpecificConfig for AAC-LC: object type 2, rate index, channel count. */
+        fun audioSpecificConfig(sampleRate: Int, channels: Int): ByteArray {
+            val freq = FREQUENCIES.indexOf(sampleRate).let { if (it < 0) 3 else it }
+            return byteArrayOf(
+                ((2 shl 3) or (freq shr 1)).toByte(),
+                (((freq and 1) shl 7) or (channels shl 3)).toByte(),
+            )
+        }
     }
 }

gearslip/src/main/java/app/seb3thehacker/gearslip/audio/AudioLink.kt

@@ -251,6 +251,12 @@ class AudioLink(
             MSG_START,
             Protobuf.varintField(1, sessionId.toLong()) + Protobuf.varintField(2, configIndex.toLong()),
         )
+        // Raw AAC-LC has no header of its own: the car needs the codec config before the first frame.
+        if (service.codecType == ServiceDiscovery.CODEC_AAC_LC) {
+            val asc = AacEncoder.audioSpecificConfig(sampleRate, channels)
+            sendOnChannel(MSG_CODEC_CONFIG, asc)
+            GearslipLog.hex("$tag: -> CodecConfig (AudioSpecificConfig)", asc)
+        }
         inFlight = 0
         lastAckAt = System.currentTimeMillis()
         if (aacSink && aac == null) {
@@ -364,6 +370,7 @@ class AudioLink(
         const val MSG_FOCUS_RESPONSE = 19
         private const val MSG_CHANNEL_OPEN_RESPONSE = 8
         private const val MSG_DATA = 0
+        private const val MSG_CODEC_CONFIG = 1
         private const val MSG_SETUP = 32768
         private const val MSG_START = 32769
         private const val MSG_STOP = 32770