feat(clients): add aux outgoing stream (mic + second input device) on Windows + macOS

Lets a user transmit a second hardware input device (e.g. line-in / aux)
alongside the mic, with its own device picker and volume, from Audio Settings.

No core/ABI/proto changes: the aux is a VC_STREAM_AUX_DEVICE stream started
with external_feed=1 and fed via vc_stream_feed_pcm (the same external-feed
pipeline screen-audio uses). Per-kind local_streams_ already allows mic +
screen + one aux to coexist; volume is a client-side gain multiply (the core's
vc_set_input_gain is mic-only/global). Aux is always-on (core never gates
AUX_DEVICE on VAD/PTT) and is tied to the voice session.

Windows: new Audio/InputDeviceCapture.cs (WASAPI shared-mode capture from a
real input endpoint + capture-endpoint enumeration); aux section in
AudioSettingsForm.cs; lifecycle in MainForm.cs; persistence in VoiceSettings.cs.

macOS: new Audio/InputDeviceCapture.swift (AVAudioEngine input-node tap pinned
to the chosen Core Audio device + device enumeration by stable UID); aux section
in SettingsWindowController.swift; lifecycle + UserDefaults persistence in
MainWindowController.swift; file registered in project.pbxproj.

Windows verified (C# solution builds clean; aux confirmed working). macOS build
+ E2E pending a Mac.

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

clients/apple/macOS/VoiceCatMac/Windows/SettingsWindowController.swift

@@ -58,6 +58,22 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
     }()
     private let inputGainValueLabel = NSTextField(labelWithString: "100%")
 
+    // Aux input stream: a second outgoing stream from another hardware input device (e.g. line-in
+    // / aux), captured client-side. Device + volume only — aux is always-on. Persisted via
+    // MainWindowController.auxEnabled / auxDeviceUID / auxGain.
+    private let auxCheckbox = NSButton(checkboxWithTitle: "Aux input stream (second device)",
+                                       target: nil, action: nil)
+    private let auxDevicePicker = NSPopUpButton()
+    private let auxRefreshButton = NSButton()
+    private let auxGainSlider: NSSlider = {
+        let s = NSSlider(value: 100, minValue: 0, maxValue: 300, target: nil, action: nil)
+        s.numberOfTickMarks = 0
+        return s
+    }()
+    private let auxGainValueLabel = NSTextField(labelWithString: "100%")
+    private let auxDeviceLabel = NSTextField(labelWithString: "Aux device:")
+    private let auxGainLabel = NSTextField(labelWithString: "Aux volume:")
+
     // Notification feedback controls. Read/write UserDefaults with the same keys VoiceCatCore's
     // FeedbackSettings reads, so EventFeedback honours these immediately.
     private let soundsCheckbox = NSButton(checkboxWithTitle: "Event sounds", target: nil, action: nil)
@@ -94,6 +110,7 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         buildUI()
         syncFromMainController()
         loadInputDevices()
+        loadAuxDevices()
     }
 
     required init?(coder: NSCoder) { fatalError() }
@@ -175,6 +192,41 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         inputGainRow.orientation = .horizontal
         inputGainRow.spacing = 8
 
+        // ── Aux input stream ──────────────────────────────────────────────────
+        let auxHeader = NSTextField(labelWithString: "Aux input stream")
+        auxHeader.font = .boldSystemFont(ofSize: NSFont.systemFontSize)
+
+        auxCheckbox.target = self
+        auxCheckbox.action = #selector(auxEnabledChanged)
+        auxCheckbox.setAccessibilityLabel("Enable aux input stream")
+        auxCheckbox.setAccessibilityHelp("Transmit a second hardware input device alongside your microphone.")
+
+        auxDeviceLabel.setAccessibilityLabel("Aux input device")
+        auxDevicePicker.setAccessibilityLabel("Aux input device")
+        auxDevicePicker.target = self
+        auxDevicePicker.action = #selector(auxDeviceChanged)
+        auxRefreshButton.title = "↺"
+        auxRefreshButton.bezelStyle = .rounded
+        auxRefreshButton.target = self
+        auxRefreshButton.action = #selector(refreshAuxDevicesClicked)
+        auxRefreshButton.setAccessibilityLabel("Refresh aux device list")
+        auxRefreshButton.toolTip = "Refresh"
+
+        auxGainLabel.setAccessibilityLabel("Aux volume")
+        auxGainSlider.target = self
+        auxGainSlider.action = #selector(auxGainChanged)
+        auxGainSlider.setAccessibilityLabel("Aux volume")
+        auxGainSlider.setAccessibilityHelp("Volume of the aux input stream. 100% is unity.")
+        auxGainValueLabel.setAccessibilityLabel("Aux volume value")
+
+        let auxDeviceRow = NSStackView(views: [auxDeviceLabel, auxDevicePicker, auxRefreshButton])
+        auxDeviceRow.orientation = .horizontal
+        auxDeviceRow.spacing = 8
+
+        let auxGainRow = NSStackView(views: [auxGainLabel, auxGainSlider, auxGainValueLabel])
+        auxGainRow.orientation = .horizontal
+        auxGainRow.spacing = 8
+
         // Notifications
         let notificationsHeader = NSTextField(labelWithString: "Notifications")
         notificationsHeader.font = .boldSystemFont(ofSize: NSFont.systemFontSize)
@@ -198,7 +250,8 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         volumeRow.spacing = 8
 
         let stack = NSStackView(views: [inputModeRow, vadRow, inputGainRow, pttRow, deviceRow,
-                                        levelRow, notificationsHeader, soundsCheckbox, volumeRow,
+                                        levelRow, auxHeader, auxCheckbox, auxDeviceRow, auxGainRow,
+                                        notificationsHeader, soundsCheckbox, volumeRow,
                                         speechCheckbox, selfTalkCheckbox, pttSoundCheckbox])
         stack.orientation = .vertical
         stack.spacing = 12
@@ -218,6 +271,8 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
             levelMeter.widthAnchor.constraint(equalToConstant: 200),
             devicePicker.widthAnchor.constraint(greaterThanOrEqualToConstant: 180),
             soundsVolumeSlider.widthAnchor.constraint(greaterThanOrEqualToConstant: 200),
+            auxDevicePicker.widthAnchor.constraint(greaterThanOrEqualToConstant: 180),
+            auxGainSlider.widthAnchor.constraint(greaterThanOrEqualToConstant: 180),
         ])
 
         syncNotificationControls()
@@ -265,6 +320,11 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         inputGainSlider.doubleValue = Double(mc.inputGain * 100)
         updateInputGainLabel()
 
+        auxCheckbox.state = mc.auxEnabled ? .on : .off
+        auxGainSlider.doubleValue = Double(mc.auxGain * 100)
+        updateAuxGainLabel()
+        updateAuxControlsEnabled()
+
         updateConditionalControls()
     }
 
@@ -338,6 +398,65 @@ final class SettingsWindowController: NSWindowController, NSWindowDelegate {
         }
     }
 
+    // MARK: - Aux input stream actions
+
+    @objc private func auxEnabledChanged() {
+        let on = auxCheckbox.state == .on
+        updateAuxControlsEnabled()
+        mainController?.applyAuxEnabled(on)
+    }
+
+    @objc private func refreshAuxDevicesClicked() { loadAuxDevices() }
+
+    @objc private func auxDeviceChanged() {
+        let uid = auxDevicePicker.selectedItem?.representedObject as? String
+        mainController?.applyAuxDevice(uid)
+    }
+
+    @objc private func auxGainChanged() {
+        mainController?.auxGain = Float(auxGainSlider.doubleValue) / 100.0
+        updateAuxGainLabel()
+    }
+
+    private func updateAuxGainLabel() {
+        let pct = Int(auxGainSlider.doubleValue.rounded())
+        auxGainValueLabel.stringValue = "\(pct)%"
+        auxGainSlider.setAccessibilityValue("\(pct) percent")
+    }
+
+    private func updateAuxControlsEnabled() {
+        let on = auxCheckbox.state == .on
+        auxDeviceLabel.isEnabled = on
+        auxDevicePicker.isEnabled = on
+        auxRefreshButton.isEnabled = on
+        auxGainLabel.isEnabled = on
+        auxGainSlider.isEnabled = on
+        auxGainValueLabel.isEnabled = on
+    }
+
+    private func loadAuxDevices() {
+        let devices = InputDeviceEnumerator.list()
+        let prevSelected = (auxDevicePicker.selectedItem?.representedObject as? String)
+            ?? mainController?.auxDeviceUID
+        auxDevicePicker.removeAllItems()
+        for d in devices {
+            let item = NSMenuItem(title: d.name, action: nil, keyEquivalent: "")
+            item.representedObject = d.uid
+            auxDevicePicker.menu?.addItem(item)
+        }
+        if let prev = prevSelected,
+           let item = auxDevicePicker.itemArray.first(where: { ($0.representedObject as? String) == prev }) {
+            auxDevicePicker.select(item)
+        } else if let def = devices.first(where: { $0.isDefault }),
+                  let item = auxDevicePicker.itemArray.first(where: { ($0.representedObject as? String) == def.uid }) {
+            auxDevicePicker.select(item)
+        } else if auxDevicePicker.numberOfItems > 0 {
+            auxDevicePicker.selectItem(at: 0)
+        }
+        // Persist the resolved selection so a relaunch (or Join Voice) opens the same device.
+        mainController?.auxDeviceUID = auxDevicePicker.selectedItem?.representedObject as? String
+    }
+
     // MARK: - Level meter (called by MainWindowController)
 
     func updateLevel(rms: Float) {