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/apple/Sources/PunktfunkClient/ContentView.swift

@@ -25,6 +25,7 @@ struct ContentView: View {
     @AppStorage(DefaultsKey.compositor) private var compositor = 0
     @AppStorage(DefaultsKey.gamepadType) private var gamepadType = 0
     @AppStorage(DefaultsKey.bitrateKbps) private var bitrateKbps = 0
+    @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
@@ -252,6 +253,7 @@ struct ContentView: View {
                 setting: PunktfunkConnection.GamepadType(
                     rawValue: UInt32(clamping: gamepadType)) ?? .auto),
             bitrateKbps: UInt32(clamping: bitrateKbps),
+            audioChannels: UInt8(clamping: audioChannels),
             launchID: launchID,
             allowTofu: host.pinnedSHA256 == nil)
     }
@@ -351,6 +353,7 @@ struct ContentView: View {
             compositor: pref,
             gamepad: pad,
             bitrateKbps: bitrate,
+            audioChannels: UInt8(clamping: audioChannels),
             autoTrust: true)
     }
 }

clients/apple/Sources/PunktfunkClient/SessionModel.swift

@@ -99,6 +99,7 @@ final class SessionModel: ObservableObject {
                  compositor: PunktfunkConnection.Compositor = .auto,
                  gamepad: PunktfunkConnection.GamepadType = .auto,
                  bitrateKbps: UInt32 = 0,
+                 audioChannels: UInt8 = 2,
                  hdrEnabled: Bool = true,
                  launchID: String? = nil,
                  allowTofu: Bool = false,
@@ -137,7 +138,7 @@ final class SessionModel: ObservableObject {
                 width: width, height: height, refreshHz: hz,
                 pinSHA256: pin, identity: identity, compositor: compositor,
                 gamepad: gamepad, bitrateKbps: bitrateKbps, videoCaps: videoCaps,
-                launchID: launchID) }
+                audioChannels: audioChannels, launchID: launchID) }
             await MainActor.run { [weak self] in
                 guard let self else { return }
                 // The user may have abandoned this attempt (window closed, another host

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -25,6 +25,7 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.libraryEnabled) private var libraryEnabled = false
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.micEnabled) private var micEnabled = true
+    @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
     @ObservedObject private var gamepads = GamepadManager.shared
@@ -173,6 +174,10 @@ struct SettingsView: View {
                 TVSelectionRow(title: "Stream mode", options: options, selection: modeTag)
                 TVSelectionRow(
                     title: "Bitrate", options: bitrateOptions, selection: $bitrateKbps)
+                TVSelectionRow(
+                    title: "Audio channels",
+                    options: [("Stereo", 2), ("5.1 Surround", 6), ("7.1 Surround", 8)],
+                    selection: $audioChannels)
                 if bitrateKbps > 1_000_000 {
                     Label(Self.gigabitWarning, systemImage: "exclamationmark.triangle.fill")
                         .font(.caption)
@@ -271,6 +276,11 @@ struct SettingsView: View {
 
     @ViewBuilder private var audioSection: some View {
         Section {
+            Picker("Audio channels", selection: $audioChannels) {
+                Text("Stereo").tag(2)
+                Text("5.1 Surround").tag(6)
+                Text("7.1 Surround").tag(8)
+            }
             #if os(macOS)
             Picker("Speaker", selection: $speakerUID) {
                 Text("System default").tag("")

clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift

@@ -15,6 +15,9 @@ public enum DefaultsKey {
     public static let gamepadType = "punktfunk.gamepadType"
     public static let gamepadID = "punktfunk.gamepadID"
     public static let bitrateKbps = "punktfunk.bitrateKbps"
+    /// 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 in-core decode + AVAudioEngine layout.
+    public static let audioChannels = "punktfunk.audioChannels"
     public static let micEnabled = "punktfunk.micEnabled"
     public static let speakerUID = "punktfunk.speakerUID"
     public static let micUID = "punktfunk.micUID"

clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift

@@ -235,6 +235,12 @@ public final class PunktfunkConnection {
     /// drain `nextHdrMeta`.
     public var isHDR: Bool { colorTransfer == 16 || colorTransfer == 18 }
 
+    /// The audio channel count the host resolved for this session (the Welcome's echo of the
+    /// requested `audioChannels`, clamped to what the host can capture): `2` (stereo), `6` (5.1)
+    /// or `8` (7.1). Build the playback layout from THIS, never the request. `2` for an older host.
+    /// PCM from `nextAudioPcm` is interleaved in the canonical wire order FL FR FC LFE RL RR SL SR.
+    public private(set) var resolvedAudioChannels: UInt8 = 2
+
     /// Connect and start a session at the requested mode (the host creates a native virtual
     /// output at exactly this size/refresh). Blocks up to `timeoutMs`.
     ///
@@ -264,6 +270,7 @@ public final class PunktfunkConnection {
         gamepad: GamepadType = .auto,
         bitrateKbps: UInt32 = 0,
         videoCaps: UInt8 = 0,
+        audioChannels: UInt8 = 2,
         launchID: String? = nil,
         timeoutMs: UInt32 = 10_000
     ) throws {
@@ -279,16 +286,16 @@ public final class PunktfunkConnection {
                     withOptionalCString(launchID) { launch in
                         if let pin = pinSHA256 {
                             return pin.withUnsafeBytes { p in
-                                punktfunk_connect_ex5(
+                                punktfunk_connect_ex6(
                                     cs, port, width, height, refreshHz, compositor.rawValue,
-                                    gamepad.rawValue, bitrateKbps, videoCaps, launch,
+                                    gamepad.rawValue, bitrateKbps, videoCaps, audioChannels, launch,
                                     p.bindMemory(to: UInt8.self).baseAddress, &observed,
                                     cert, key, timeoutMs)
                             }
                         }
-                        return punktfunk_connect_ex5(
+                        return punktfunk_connect_ex6(
                             cs, port, width, height, refreshHz, compositor.rawValue,
-                            gamepad.rawValue, bitrateKbps, videoCaps, launch,
+                            gamepad.rawValue, bitrateKbps, videoCaps, audioChannels, launch,
                             nil, &observed, cert, key, timeoutMs)
                     }
                 }
@@ -320,6 +327,9 @@ public final class PunktfunkConnection {
         colorMatrix = mtx
         colorFullRange = fullRange != 0
         bitDepth = depth
+        var ac: UInt8 = 2
+        _ = punktfunk_connection_audio_channels(handle, &ac)
+        resolvedAudioChannels = ac
     }
 
     /// A bandwidth speed-test measurement (see `startSpeedTest`). Partial until `done`.
@@ -468,6 +478,50 @@ public final class PunktfunkConnection {
         }
     }
 
+    /// One decoded audio frame from `nextAudioPcm`: interleaved 32-bit float at 48 kHz, in the
+    /// canonical wire channel order FL FR FC LFE RL RR SL SR (the first `channels`).
+    public struct AudioPCM: Sendable {
+        /// Interleaved f32 samples (`frameCount * channels` long), wire channel order.
+        public let samples: [Float]
+        /// Samples per channel.
+        public let frameCount: Int
+        /// Channel count (2/6/8) — `resolvedAudioChannels`.
+        public let channels: Int
+        public let ptsNs: UInt64
+        public let seq: UInt32
+    }
+
+    /// Pull the next audio frame, **decoded in-core** to interleaved f32 PCM — Apple's AudioToolbox
+    /// Opus path is stereo-only, so surround (and, for uniformity, stereo too) is decoded by the
+    /// Rust core (libopus multistream) and handed back as PCM. nil on timeout, throws `.closed` once
+    /// the session ended. Drain from a dedicated audio thread (do NOT also call `nextAudio` — they
+    /// share the underlying queue). The returned `samples` are copied out, so the buffer is owned.
+    public func nextAudioPcm(timeoutMs: UInt32 = 100) throws -> AudioPCM? {
+        audioLock.lock()
+        defer { audioLock.unlock() }
+        guard let h = liveHandle() else { throw PunktfunkClientError.closed }
+
+        var out = PunktfunkAudioPcm()
+        let rc = punktfunk_connection_next_audio_pcm(h, &out, timeoutMs)
+        switch rc {
+        case statusOK:
+            let channels = Int(out.channels)
+            let total = Int(out.frame_count) * channels
+            guard let base = out.samples, total > 0 else { return nil }
+            // Copy: the pointer borrows connection memory only until the next PCM call.
+            let samples = Array(UnsafeBufferPointer(start: base, count: total))
+            return AudioPCM(
+                samples: samples, frameCount: Int(out.frame_count),
+                channels: channels, ptsNs: out.pts_ns, seq: out.seq)
+        case statusNoFrame:
+            return nil
+        case statusClosed:
+            throw PunktfunkClientError.closed
+        default:
+            throw PunktfunkClientError.status(rc)
+        }
+    }
+
     /// Pull the next force-feedback update for the GCController haptics engine:
     /// `(pad, lowFrequency, highFrequency)` with 0...0xFFFF amplitudes, (0, 0) = stop.
     /// Drain from the (single) feedback thread, alongside `nextHidOutput`.

clients/apple/Sources/PunktfunkKit/SessionAudio.swift

@@ -19,13 +19,13 @@ import os
 
 private let log = Logger(subsystem: "io.unom.punktfunk", category: "audio")
 
-/// SPSC-ish jitter ring (interleaved stereo float), drain thread → render callback.
-/// The unfair lock is held for microseconds; fine at render-callback rates. Priming:
+/// SPSC-ish jitter ring (interleaved float, `channels` per frame), drain thread → render
+/// callback. The unfair lock is held for microseconds; fine at render-callback rates. Priming:
 /// reads return silence until enough is buffered (at least `prefill`, and at least one
 /// packet more than the device's render quantum — large-buffer devices would otherwise
 /// chronically out-demand the prefill and oscillate prime → dropout → re-prime), and an
 /// underrun re-primes, concealing jitter as one short dip instead of sustained crackle.
-/// All counts stay even (whole stereo frames), so L/R interleave can never flip.
+/// All counts stay whole frames (multiples of `channels`), so the interleave can never slip.
 final class AudioRing: @unchecked Sendable {
     private var buf: [Float]
     private var readIdx = 0
@@ -34,12 +34,14 @@ final class AudioRing: @unchecked Sendable {
     private var renderQuantum = 0
     private let prefill: Int
     private let highWater: Int
+    private let channels: Int
     private let lock = OSAllocatedUnfairLock()
 
-    /// `capacity`/`prefill` in samples (interleaved — 2 per frame, both must be even).
-    init(capacity: Int, prefill: Int) {
+    /// `capacity`/`prefill` in samples (interleaved — `channels` per frame, both whole frames).
+    init(capacity: Int, prefill: Int, channels: Int) {
         buf = [Float](repeating: 0, count: capacity)
         self.prefill = prefill
+        self.channels = channels
         highWater = prefill * 4
     }
 
@@ -74,8 +76,8 @@ final class AudioRing: @unchecked Sendable {
         renderQuantum = max(renderQuantum, count)
         let available = writeIdx - readIdx
         if !primed {
-            // 480 samples = one 5 ms host packet of slack beyond the device's demand.
-            if available >= max(prefill, renderQuantum + 480) {
+            // One 5 ms host packet (240 frames × channels) of slack beyond the device's demand.
+            if available >= max(prefill, renderQuantum + 240 * channels) {
                 primed = true
             } else {
                 for i in 0..<count { out[i] = 0 }
@@ -113,10 +115,55 @@ private final class StopFlag: @unchecked Sendable {
 /// Render-block-owned scratch storage: freed exactly when the closure (and thus the
 /// last possible render call) is released — never racing CoreAudio.
 private final class ScratchBuffer {
-    let ptr = UnsafeMutablePointer<Float>.allocate(capacity: 8192 * 2)
+    // 8192 frames × up to 8 channels (7.1) — the render block caps `frames` at 8192.
+    let ptr = UnsafeMutablePointer<Float>.allocate(capacity: 8192 * 8)
     deinit { ptr.deallocate() }
 }
 
+/// CoreAudio channel layout for the canonical wire order FL FR FC LFE RL RR [SL SR]. nil for
+/// stereo (the standard layout is correct). For 5.1/7.1 we list explicit channel labels via
+/// `kAudioChannelLayoutTag_UseChannelDescriptions` — preset tags (DTS_5_1 etc.) don't reliably
+/// match Moonlight's order. NB the 7.1 mapping (verified against the WASAPI 0x63F + SPA orderings):
+/// wire idx 4-5 = RL/RR = the WAVE *back* pair → LeftSurround/RightSurround; idx 6-7 = SL/SR = the
+/// WAVE *side* pair → LeftSurroundDirect/RightSurroundDirect. (Using RearSurround* for 6-7 would
+/// swap side/back vs the Windows/Linux clients.)
+private func wireChannelLayout(channels: Int) -> AVAudioChannelLayout? {
+    let labels: [AudioChannelLabel]
+    switch channels {
+    case 6:
+        labels = [
+            kAudioChannelLabel_Left, kAudioChannelLabel_Right, kAudioChannelLabel_Center,
+            kAudioChannelLabel_LFEScreen, kAudioChannelLabel_LeftSurround,
+            kAudioChannelLabel_RightSurround,
+        ]
+    case 8:
+        labels = [
+            kAudioChannelLabel_Left, kAudioChannelLabel_Right, kAudioChannelLabel_Center,
+            kAudioChannelLabel_LFEScreen,
+            kAudioChannelLabel_LeftSurround, kAudioChannelLabel_RightSurround, // wire RL/RR (back)
+            kAudioChannelLabel_LeftSurroundDirect, kAudioChannelLabel_RightSurroundDirect, // wire SL/SR (side)
+        ]
+    default:
+        return nil
+    }
+    let size = MemoryLayout<AudioChannelLayout>.size
+        + (labels.count - 1) * MemoryLayout<AudioChannelDescription>.stride
+    let raw = UnsafeMutableRawPointer.allocate(byteCount: size, alignment: 16)
+    defer { raw.deallocate() }
+    let layout = raw.bindMemory(to: AudioChannelLayout.self, capacity: 1)
+    layout.pointee.mChannelLayoutTag = kAudioChannelLayoutTag_UseChannelDescriptions
+    layout.pointee.mChannelBitmap = AudioChannelBitmap(rawValue: 0)
+    layout.pointee.mNumberChannelDescriptions = UInt32(labels.count)
+    let descs = UnsafeMutableBufferPointer(
+        start: &layout.pointee.mChannelDescriptions, count: labels.count)
+    for (i, lbl) in labels.enumerated() {
+        descs[i] = AudioChannelDescription(
+            mChannelLabel: lbl, mChannelFlags: AudioChannelFlags(rawValue: 0),
+            mCoordinates: (0, 0, 0))
+    }
+    return AVAudioChannelLayout(layout: layout)
+}
+
 public final class SessionAudio {
     private let connection: PunktfunkConnection
     private let flag = StopFlag()
@@ -229,9 +276,13 @@ public final class SessionAudio {
     // MARK: - Playback (host → speaker)
 
     private func startPlayback(speakerUID: String) {
-        // 1 s of interleaved stereo capacity, ~20 ms prefill: four 5 ms host packets of
-        // jitter absorption before the first sample plays.
-        let ring = AudioRing(capacity: 96_000, prefill: 1920)
+        // Build the playback layout from the host-RESOLVED channel count (never the request):
+        // 2 = stereo / 6 = 5.1 / 8 = 7.1, canonical wire order FL FR FC LFE RL RR SL SR.
+        let channels = Int(connection.resolvedAudioChannels)
+        // 1 s interleaved capacity, ~20 ms prefill (four 5 ms host packets of jitter absorption
+        // before the first sample plays), both scaled by the channel count.
+        let ring = AudioRing(
+            capacity: 48_000 * channels, prefill: 960 * channels, channels: channels)
 
         let engine = AVAudioEngine()
         #if os(macOS)
@@ -247,21 +298,32 @@ public final class SessionAudio {
         }
         #endif
 
-        // Engine-native deinterleaved float; the render block deinterleaves from the ring.
-        guard let format = AVAudioFormat(standardFormatWithSampleRate: 48_000, channels: 2)
-        else { return }
+        // Engine-native deinterleaved float; the render block deinterleaves from the ring. Surround
+        // uses an explicit wire-order channel layout; the mixer downmixes to the output device when
+        // it has fewer speakers (e.g. an iPhone's stereo built-ins). (Explicit if/else rather than
+        // map/flatMap so it's correct whether the channelLayout initializer is failable or not.)
+        var format: AVAudioFormat?
+        if channels == 2 {
+            format = AVAudioFormat(standardFormatWithSampleRate: 48_000, channels: 2)
+        } else if let layout = wireChannelLayout(channels: channels) {
+            format = AVAudioFormat(standardFormatWithSampleRate: 48_000, channelLayout: layout)
+        }
+        guard let format else {
+            log.error("could not build \(channels)-channel audio format — audio disabled")
+            return
+        }
         let scratch = ScratchBuffer() // block-owned; freed with the closure
         let source = AVAudioSourceNode(format: format) { _, _, frameCount, abl -> OSStatus in
             let frames = Int(frameCount)
             guard frames <= 8192 else { return kAudioUnitErr_TooManyFramesToProcess }
-            ring.read(into: scratch.ptr, count: frames * 2)
+            ring.read(into: scratch.ptr, count: frames * channels)
             let buffers = UnsafeMutableAudioBufferListPointer(abl)
-            if buffers.count >= 2,
-               let left = buffers[0].mData?.assumingMemoryBound(to: Float.self),
-               let right = buffers[1].mData?.assumingMemoryBound(to: Float.self) {
-                for f in 0..<frames {
-                    left[f] = scratch.ptr[f * 2]
-                    right[f] = scratch.ptr[f * 2 + 1]
+            // Deinterleave the wire-order interleaved ring into the engine's per-channel buses.
+            if buffers.count >= channels {
+                for ch in 0..<channels {
+                    if let dst = buffers[ch].mData?.assumingMemoryBound(to: Float.self) {
+                        for f in 0..<frames { dst[f] = scratch.ptr[f * channels + ch] }
+                    }
                 }
             }
             return noErr
@@ -292,29 +354,20 @@ public final class SessionAudio {
         stateLock.unlock()
         let thread = Thread { [connection, flag, drainDone] in
             defer { drainDone.signal() }
-            guard let decoder = try? OpusDecoder(framesPerPacket: 240),
-                  let pcm = AVAudioPCMBuffer(
-                      pcmFormat: decoder.pcmFormat, frameCapacity: 5760)
-            else {
-                log.error("Opus decoder unavailable — audio playback disabled")
-                return
-            }
+            // Decode happens IN-CORE (libopus multistream) — AudioToolbox's Opus path is
+            // stereo-only — and is handed back as interleaved f32 PCM in wire channel order.
             while !flag.isStopped {
-                let packet: AudioPacket?
+                let pcm: PunktfunkConnection.AudioPCM?
                 do {
-                    packet = try connection.nextAudio(timeoutMs: 100)
+                    pcm = try connection.nextAudioPcm(timeoutMs: 100)
                 } catch {
                     break // session closed
                 }
-                guard let packet else { continue }
-                do {
-                    let frames = try decoder.decode(packet.data, into: pcm)
-                    if frames > 0, let p = pcm.floatChannelData?[0] {
-                        ring.write(p, count: Int(frames) * 2)
+                guard let pcm, pcm.frameCount > 0 else { continue }
+                pcm.samples.withUnsafeBufferPointer { p in
+                    if let base = p.baseAddress {
+                        ring.write(base, count: pcm.frameCount * pcm.channels)
                     }
-                } catch {
-                    // One corrupt packet ≠ a dead stream; skip it.
-                    log.warning("audio decode failed: \(error.localizedDescription)")
                 }
             }
         }