Skeleton scaffolding for the iOS VoiceProcessingIO backend that
will replace cpal on iOS. This commit lands the dependency + the
module + a constructable AudioUnit that emits silence and drops
input; nothing in engine.rs is wired up yet (that is commit 2).
Compilation contract for this commit:
* Linux / Windows / macOS / Android builds unaffected (the new
module is target_os='ios' gated, the new dep is in
'[target."cfg(target_os = \"ios\")"]').
* iOS build pulls in coreaudio-rs 0.14, constructs a VPIO unit,
pins stream format to 48 kHz Int16 mono on both buses, installs
no-op input + silence-emitting render callbacks, initializes,
and starts. No audio is actually moved until commits 3/4.
Why VPIO and not RemoteIO via cpal: cpal's iOS backend opens
RemoteIO with no control over stream format / buffer size /
channels and produces a mono-only output element that stays bound
to the route present at construction time. End-user symptom on
iPhone 16 Pro iOS 18.7.8: tapping Speaker in the picker flips
AVAudioSession.currentRoute.outputs to Speaker (confirmed in our
diagnostic logs from commit da631a2) but audio keeps coming out
the receiver because the AudioUnit's output binding is stale.
Every production iOS VoIP client (Mumble iOS, Linphone /
mediastreamer2, Signal-iOS, Jitsi Meet iOS, the WebRTC reference
impl) avoids RemoteIO and uses VoiceProcessingIO instead. VPIO is
Apple's recommended voice unit; it ships hardware AEC + AGC + NS
and re-binds the physical transducer correctly on route changes
because it IS the canonical voice unit on iOS — FaceTime's audio
path runs through it.
coreaudio-rs 0.14 (RustAudio org, 8.6M downloads, same maintainers
as cpal) gives us a safe wrapper around the AudioUnit C API on
iOS. Uses objc2-* crates underneath so links cleanly into iOS
builds. ios_voice_unit.rs sits next to sdl_output.rs as the iOS
sibling of the Linux SDL2 output path.
Build verify (Linux host): `cargo check -p chanora_audio`
finished clean in 16.09s. iOS build verification happens in
commit 2 when the module is exercised; for this commit the module
compiles in isolation but is dead code on iOS too (no caller).
EdisonJwa
af686ca7a6