feat(audio): end-to-end 5.1/7.1 surround across the native path + all clients

Adds negotiated 5.1/7.1 surround to the punktfunk/1 protocol and every client
(previously stereo-only):

- core: new shared `audio` layout table (LAYOUT_51/71 + identity multistream
  mapping, canonical wire order FL FR FC LFE RL RR SL SR); Hello/Welcome
  `audio_channels` negotiation via the trailing-byte back-compat pattern (old
  peers fall back to stereo); C-ABI `punktfunk_connect_ex6`,
  `punktfunk_connection_audio_channels`, and in-core multistream decode
  `punktfunk_connection_next_audio_pcm` for embedders without a multistream
  Opus decoder. Real-libopus channel-identity round-trip test.
- host: native audio thread captures + Opus-(multi)stream-encodes at the
  negotiated count (with a cross-session cached-capturer channel-mismatch fix);
  GameStream surround unified onto the safe `opus::MSEncoder`, dropping
  `audiopus_sys` (~4 unsafe blocks) and un-gating Windows GameStream surround;
  WASAPI loopback capture relaxed to 2/6/8 with the correct dwChannelMask.
- clients: Linux (PipeWire), Windows (WASAPI), Android (AAudio) decode via
  `opus::MSDecoder` + render multichannel; Apple decodes in-core to PCM →
  AVAudioEngine with an explicit wire-order channel layout; each gains a
  Stereo/5.1/7.1 setting. `punktfunk-probe --audio-channels N` is the headless
  validator.

Verified on Linux: core/host/linux/probe test suites + the Android Rust
(cargo-ndk) build, clippy -D warnings, and rustfmt all green. Windows/Apple
builds, all on-glass checks, and the live native loopback are pending (CI / a
free box).

Also lands the concurrent in-tree HEVC 4:4:4 host work (PUNKTFUNK_444): it
shares the same touched files (quic.rs, punktfunk1.rs, encode/*, ...) and so
cannot be committed separately from the surround changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/android/app/src/main/kotlin/io/unom/punktfunk/ConnectScreen.kt

@@ -163,7 +163,7 @@ fun ConnectScreen(settings: Settings, onConnected: (Long) -> Unit) {
                     targetHost, targetPort, w, h, hz,
                     id.certPem, id.privateKeyPem, pinHex ?: "",
                     settings.bitrateKbps, settings.compositor, gamepadPref,
-                    hdrEnabled,
+                    hdrEnabled, settings.audioChannels,
                 )
             }
             connecting = false

clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt

@@ -16,6 +16,9 @@ data class Settings(
     val bitrateKbps: Int = 0,
     val compositor: Int = 0,
     val gamepad: Int = 0,
+    /** Requested audio channel count: 2 (stereo), 6 (5.1) or 8 (7.1). The host clamps to what it
+     * can capture; the resolved count drives the decoder + AAudio layout. */
+    val audioChannels: Int = 2,
     val micEnabled: Boolean = false,
     /** Show the live stats overlay (FPS / throughput / latency) during a stream. */
     val statsHudEnabled: Boolean = true,
@@ -39,6 +42,7 @@ class SettingsStore(context: Context) {
         bitrateKbps = prefs.getInt(K_BITRATE, 0),
         compositor = prefs.getInt(K_COMPOSITOR, 0),
         gamepad = prefs.getInt(K_GAMEPAD, 0),
+        audioChannels = prefs.getInt(K_AUDIO_CH, 2),
         micEnabled = prefs.getBoolean(K_MIC, false),
         statsHudEnabled = prefs.getBoolean(K_HUD, true),
         trackpadMode = prefs.getBoolean(K_TRACKPAD, true),
@@ -52,6 +56,7 @@ class SettingsStore(context: Context) {
             .putInt(K_BITRATE, s.bitrateKbps)
             .putInt(K_COMPOSITOR, s.compositor)
             .putInt(K_GAMEPAD, s.gamepad)
+            .putInt(K_AUDIO_CH, s.audioChannels)
             .putBoolean(K_MIC, s.micEnabled)
             .putBoolean(K_HUD, s.statsHudEnabled)
             .putBoolean(K_TRACKPAD, s.trackpadMode)
@@ -65,6 +70,7 @@ class SettingsStore(context: Context) {
         const val K_BITRATE = "bitrate_kbps"
         const val K_COMPOSITOR = "compositor"
         const val K_GAMEPAD = "gamepad"
+        const val K_AUDIO_CH = "audio_channels"
         const val K_MIC = "mic_enabled"
         const val K_HUD = "stats_hud_enabled"
         const val K_TRACKPAD = "trackpad_mode"
@@ -133,6 +139,13 @@ val REFRESH_OPTIONS = listOf(
     240 to "240 Hz",
 )
 
+/** (channel count, label). 2 = stereo (default), 6 = 5.1, 8 = 7.1. */
+val AUDIO_CHANNEL_OPTIONS = listOf(
+    2 to "Stereo",
+    6 to "5.1 Surround",
+    8 to "7.1 Surround",
+)
+
 /** (kbps, label). `0` = host default. */
 val BITRATE_OPTIONS = listOf(
     0 to "Automatic",

clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt

@@ -104,6 +104,12 @@ fun SettingsScreen(initial: Settings, onChange: (Settings) -> Unit, onBack: () -
         }
 
         SettingsGroup("Audio") {
+            SettingDropdown(
+                label = "Audio channels",
+                options = AUDIO_CHANNEL_OPTIONS,
+                selected = s.audioChannels,
+            ) { ch -> update(s.copy(audioChannels = ch)) }
+
             ToggleRow(
                 title = "Microphone",
                 subtitle = "Send your mic to the host's virtual microphone",