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>

PROGRESS.md

@@ -10,6 +10,30 @@ up instantly. Newest status at the top.
 
 ## ▶ Where we left off / next action
 
+- **Done (2026-06-23):** **Aux outgoing stream (mic + a second input device) — Windows + macOS.**
+  Users can now 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`, captured client-side, 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). The aux is always-on (the core never gates `AUX_DEVICE` on
+  VAD/PTT) and is tied to the voice session (started on Join Voice when enabled, stopped on Leave).
+  - **Windows:** new `Audio/InputDeviceCapture.cs` (WASAPI shared-mode capture from a real input
+    endpoint via `IMMDevice.Activate(IAudioClient)`, 48 kHz/s16, 20 ms frames) + `InputDeviceEnumerator`
+    (WASAPI capture-endpoint list — separate from the core's miniaudio ids). Aux section in
+    `AudioSettingsForm.cs` (enable checkbox, device combo, refresh, volume slider, accessible names,
+    live-apply + Cancel revert via callbacks). Lifecycle in `MainForm.cs` (`_auxStreamId` +
+    `InputDeviceCapture`). Persisted in `VoiceSettings.cs` (`AuxEnabled/AuxDeviceId/AuxGain`).
+  - **macOS:** new `Audio/InputDeviceCapture.swift` (AVAudioEngine input-node tap pinned to the chosen
+    Core Audio device via `kAudioOutputUnitProperty_CurrentDevice`; AVAudioConverter → 48 kHz int16;
+    20 ms framing modelled on `ScreenAudioCapture`) + `InputDeviceEnumerator` (Core Audio device list
+    by stable UID). Aux section in `SettingsWindowController.swift`; lifecycle + UserDefaults
+    persistence (`voice.aux*`) in `MainWindowController.swift`. New file added to `project.pbxproj`.
+  - **Verify status:** Windows C# solution builds clean (0 warn/0 err); `ctest` core suite unchanged
+    (no core edits). **Next (manual):** on a Mac, build `VoiceCatMac.xcodeproj`; then two-client E2E —
+    enable aux on a second input device, confirm two distinct streams for the sender and that the aux
+    volume slider moves the aux level independently of the mic; confirm persistence across relaunch.
+
 - **Done (2026-06-23):** **Input-settings persistence, mic input gain, + two iOS bugs (all 3
   clients).** Four fixes:
   1. **Input settings now persist.** Transmission mode (VAD/PTT/Always-On), VAD threshold, and the