fix(voice,ios): scope AVAudioSession VoiceChat to call lifetime (#33)

Adopt a call-scoped VoIP audio session lifecycle so other apps' audio is
not stopped while Chanora is idle and the in-call session does not get
clobbered by media-server resets unrelated to voice.

AppDelegate.swift
- Set .ambient + .mixWithOthers as the idle baseline so the app does not
  hold a VoiceChat session when no call is active.
- Switch to .playAndRecord + .voiceChat + .mixWithOthers + .duckOthers
  on demand via the new chanora/ios_audio_session MethodChannel, and
  revert to .ambient on deactivate.
- Gate the media-services-reset rebuild on voiceSessionActive so a
  stray reset during idle no longer reactivates VoiceChat.

ios_audio_session_controller.dart (new)
- Thin Dart wrapper around chanora/ios_audio_session with activate/
  deactivate; no-op on non-iOS; swallows PlatformException to keep
  audio start/stop resilient to platform-side races.

main.dart
- Activate the iOS audio session on BridgeEvent_AudioStarted, deactivate
  on BridgeEvent_AudioStopped, fire-and-forget via unawaited().

ios_voice_unit.rs
- Add the 10 local bindings required by the render-callback closure
  preamble (wav_recorder_for_render, render_recorder_active,
  render_ref_len, render_ref_accum, cb_count, last_num_frames,
  num_frames_changes, callbacks_with_audio, callbacks_with_silence)
  so the iOS target compiles cleanly with the new lifecycle wiring.

Tests
- 5 unit tests in test/services/ios_audio_session_controller_test.dart
  cover activate/deactivate on iOS, no-op on non-iOS, and graceful
  PlatformException handling.

Docs
- SRS SRS-110 expanded to cover the call-scoped lifecycle invariant.
- SysDes mobile-voice row updated to reflect the MethodChannel and
  .ambient idle baseline.
- implementation-status-2026-05-28 voiceChat row flipped to done.

Verification
- flutter analyze: No issues found (2.8s)
- flutter test: 195 passed / 2 skipped / 0 failed
- cargo build -p chanora_audio --target aarch64-apple-ios: clean
- cargo build -p chanora_audio (macOS host): clean

Device QA matrix (Spotify-keeps-playing-while-idle, mix-during-call,
revert-on-call-end, media-services-reset-during-idle) remains pending
on physical hardware.

crates/chanora_audio/src/ios_voice_unit.rs

@@ -989,9 +989,26 @@ impl IosVoiceUnit {
         let output_gain_for_render = params.output_gain.clone();
         let output_muted_for_render = params.output_muted.clone();
         let audio_processing_stats_for_render = params.audio_processing_stats.clone();
+        let wav_recorder_for_render = wav_recorder.clone();
         // Level meter decimation: the render callback fires ~93
         // times/sec, but the bridge consumer reads at ~30 Hz.
         let mut render_level_decimation: u32 = 0;
+        // Render-side reference recorder state. Accumulates downmixed
+        // mono samples until a 10 ms frame is full, then pushes to the
+        // recorder. `render_recorder_active` tracks whether the
+        // recorder is currently armed so we can reset the accumulator
+        // when it goes from off→on (avoids stitching pre-stop tail
+        // into the post-start head).
+        let mut render_recorder_active: bool = false;
+        let mut render_ref_len: usize = 0;
+        let mut render_ref_accum: [f32; crate::frame::FRAME_10MS_SAMPLES] =
+            [0.0; crate::frame::FRAME_10MS_SAMPLES];
+        // Diagnostic counters sampled every 100 callbacks.
+        let mut cb_count: u64 = 0;
+        let mut last_num_frames: usize = 0;
+        let mut num_frames_changes: u64 = 0;
+        let mut callbacks_with_audio: u64 = 0;
+        let mut callbacks_with_silence: u64 = 0;
         unit.set_render_callback(move |args: render_callback::Args<data::Interleaved<i16>>| {
             let render_callback::Args {
                 data,