fix(audio,ios): correct ios_voice_unit imports + iOS stop() field access (rc.8+61)

iOS build of chanora_audio (target aarch64-apple-ios) failed with
four compilation errors after commit 2 landed. Root causes were
all simple symbol-path / cfg-gating mistakes from the skeleton
commit; the underlying design is unchanged.

1. ios_voice_unit.rs: wrong import path for OutPacket. The
   chanora_protocol crate re-exports it at the crate root
   (`pub use ...::OutPacket` in lib.rs line 52), not from a
   `voice` submodule (which doesn't exist).
   - use chanora_protocol::voice::OutPacket;
   + use chanora_protocol::OutPacket;

2. ios_voice_unit.rs: LinearPcmFlags lives in
   `coreaudio::audio_unit::audio_format`, not in
   `stream_format` (the doc page lists it under StreamFormat but
   the actual module path is the upstream Apple naming).
   - use coreaudio::audio_unit::stream_format::LinearPcmFlags;
   + use coreaudio::audio_unit::audio_format::LinearPcmFlags;

3. ios_voice_unit.rs: `Ordering` import unused (commit 1
   skeleton callbacks don't load atomics yet — that comes in
   commits 3 + 4). Remove from the std::sync::atomic import to
   silence the unused_imports warning.

4. engine.rs::AudioEngine::stop(): the existing body unconditionally
   touched self._input_stream and self._output_stream, but commit
   2 cfg-gated those fields away on iOS (and added an iOS-only
   _ios_voice_unit field in their place). Split the field drop
   logic with the same target_os = "ios" cfg so each platform
   only touches the fields it actually has.

Also clean up two pre-existing warnings exposed by the iOS cfg
gating:

5. engine.rs: `tracing::{error, warn}` were imported
   unconditionally but are only used inside cpal log lines.
   Cfg-gate the import to not(target_os = "ios").
6. engine.rs: `AudioData`, `CodecType`, `OutAudio` from
   chanora_protocol are only referenced in the Opus encoder feed
   inside CaptureState — cpal-side only. Cfg-gate to
   not(target_os = "ios"); keep `InboundVoice` + `OutPacket`
   on the unconditional path because the inbound forwarder + (in
   commit 3) the iOS capture pipeline both reference them.

Also fix a stray duplicate `#[cfg(not(target_os = "ios"))]`
attribute that landed on line 18 in commit 2.

Build verify (Linux host): cargo check -p chanora_audio clean
in 0.53s. iOS-side check pending on Mac.

Build counter bumped 60 -> 61 — the About dialog will display
v1.0.0-rc.8+61 so the user can confirm the build under test
matches this commit.

crates/chanora_audio/src/ios_voice_unit.rs

@@ -75,14 +75,12 @@
 //! * AVAudioSession category / mode configuration — Swift owns the
 //!   session (it must be set up before Flutter loads).
 
-use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU32};
 use std::sync::{Arc, Mutex};
 
+use coreaudio::audio_unit::audio_format::LinearPcmFlags;
 use coreaudio::audio_unit::render_callback::{self, data};
-use coreaudio::audio_unit::{
-    AudioUnit, Element, SampleFormat, Scope, StreamFormat,
-};
-use coreaudio::audio_unit::stream_format::LinearPcmFlags;
+use coreaudio::audio_unit::{AudioUnit, Element, SampleFormat, Scope, StreamFormat};
 use coreaudio::audio_unit::IOType;
 use tokio::sync::mpsc;
 use tracing::{info, warn};
@@ -90,7 +88,7 @@ use tsclientlib::audio::AudioHandler;
 
 use crate::engine::SessionAudioId;
 use crate::AudioError;
-use chanora_protocol::voice::OutPacket;
+use chanora_protocol::OutPacket;
 
 /// 20 ms at 48 kHz mono — one Opus frame's worth of samples.
 /// Aligning the AudioUnit IO buffer to this frame size keeps the