feat(apple): session audio — host playback + mic uplink, device pickers in Settings

Both directions of the audio plane, on CoreAudio's built-in Opus codec
(kAudioFormatOpus — no bundled libopus; OpusCodec.swift, round trip unit-tested):

- Playback: a drain thread pulls nextAudio() packets, decodes, and writes a priming
  jitter ring feeding an AVAudioSourceNode (~20 ms prefill, adaptive to the device's
  render quantum so large-buffer devices don't oscillate prime/dropout; a high-water
  clamp sheds stall backlog so one network hiccup can't permanently lag audio behind
  video; underrun re-primes — one dip, not sustained crackle).
- Mic: a second engine taps the input device, resamples to 48 kHz stereo, Opus-encodes
  20 ms chunks and sendMic()s them into the host's virtual PipeWire source. Permission
  via AVCaptureDevice (NSMicrophoneUsageDescription added to the Xcode target).
- Settings: Speaker + Microphone pickers (CoreAudio HAL enumeration, persisted by
  device UID — "System default" leaves the engine unpinned so it follows macOS device
  changes) and a "Send microphone" toggle (default on). Applies from the next session.
- Audio starts with streaming, never during the trust prompt (no host sound — and no
  mic uplink — before the user trusted the host); teardown stops audio before close().

Adversarial-review fixes baked in: stop() and the dangling mic-permission callback
share one lock+flag protocol (no hot mic with no owner), the connect-success handler
bails when the attempt was abandoned mid-handshake (no session/mic for a dead window),
SessionAudio gets a deinit backstop (a dropped instance can't pin the connection via
its drain thread), and the render scratch buffer is block-owned (was leaked per
session).

Verified live against the box: remote test decodes 100 host Opus packets to PCM and
the host opens its virtual mic on the first uplinked frame ("punktfunk/1 virtual mic
ready"); on-glass session runs with both engines up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -11,6 +11,11 @@ struct SettingsView: View {
     @AppStorage("punktfunk.height") private var height = 1080
     @AppStorage("punktfunk.hz") private var hz = 60
     @AppStorage("punktfunk.compositor") private var compositor = 0
+    @AppStorage("punktfunk.speakerUID") private var speakerUID = ""
+    @AppStorage("punktfunk.micUID") private var micUID = ""
+    @AppStorage("punktfunk.micEnabled") private var micEnabled = true
+    @State private var outputDevices: [AudioDevice] = []
+    @State private var inputDevices: [AudioDevice] = []
 
     var body: some View {
         Form {
@@ -33,6 +38,38 @@ struct SettingsView: View {
                     .font(.caption)
                     .foregroundStyle(.secondary)
             }
+            Section {
+                Picker("Speaker", selection: $speakerUID) {
+                    Text("System default").tag("")
+                    ForEach(outputDevices) { device in
+                        Text(device.name).tag(device.uid)
+                    }
+                    if !speakerUID.isEmpty,
+                       !outputDevices.contains(where: { $0.uid == speakerUID }) {
+                        Text("Unavailable device").tag(speakerUID)
+                    }
+                }
+                Toggle("Send microphone to the host", isOn: $micEnabled)
+                Picker("Microphone", selection: $micUID) {
+                    Text("System default").tag("")
+                    ForEach(inputDevices) { device in
+                        Text(device.name).tag(device.uid)
+                    }
+                    if !micUID.isEmpty,
+                       !inputDevices.contains(where: { $0.uid == micUID }) {
+                        Text("Unavailable device").tag(micUID)
+                    }
+                }
+                .disabled(!micEnabled)
+            } header: {
+                Text("Audio")
+            } footer: {
+                Text("Host audio plays through the speaker; the microphone feeds the "
+                    + "host's virtual mic. System default follows macOS device changes. "
+                    + "Applies from the next session.")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
             Section {
                 Picker("Compositor", selection: $compositor) {
                     Text("Automatic").tag(0)
@@ -54,6 +91,10 @@ struct SettingsView: View {
         .formStyle(.grouped)
         .frame(width: 380)
         .fixedSize()
+        .onAppear {
+            outputDevices = AudioDevices.outputs()
+            inputDevices = AudioDevices.inputs()
+        }
     }
 
     private func fillFromMainScreen() {