On iOS the remote end heard the mic twice and crackly (BT headset + internal
mic in Voice Chat; mono + stereo copies of the internal mic in Stereo Mic).
Root cause is an io-thread ordering race. `ensure_audio_running()` only set
`external_capture` when a MIC stream already existed, but it also runs from
`sync_remote_streams` on the post-auth `ServerStateSnapshot` — before the user
joins voice. With `external_playback_` still false and no MIC stream,
`AudioEngine::start()` opened a real miniaudio capture device that stayed open
all session (later calls early-return on running()), racing the AVAudioEngine
input tap fed via `vc_stream_feed_pcm`. `on_capture_frame` then encoded+sent
both paths — the mic transmitted twice, the two unsynchronized capture clocks
producing the crackle.
- core (ensure_audio_running): force `external_capture = true` whenever
`external_playback_` is set, so iOS unified mode never opens a hardware
capture device. No-op on desktop.
- ios (AppState): move `setExternalPlayback(true)` before `connect()`, so the
flag is set before the io thread processes any message — closing the race.
Verified on device: remote end hears the iOS mic once and clean in both Voice
Chat (+ BT) and Stereo Mic.
Talon
b14cf2a4e8