feat(audio,android): add MobileVoiceAudioBackend + AndroidVoiceUnit (oboe-rs)

Add the cross-platform MobileVoiceAudioBackend trait, plus the
Android implementation AndroidVoiceUnit backed by oboe-rs 0.6.x.
AndroidVoiceUnit owns AAudio stream setup with VoiceCommunication
usage/preset, performance-mode LowLatency request, sharing-mode
Exclusive best-effort, hardware AEC/NS/AGC engagement via JNI, and
the diagnostics snapshot publish path used by SDD-116 evidence
collection.

Cargo.toml: adds oboe = "0.6" under the Android target.

Trace: SDD-111, SDD-112, SDD-113, SRS-210, SRS-211, SRS-212, SRS-213,
SRS-214.

crates/chanora_audio/src/lib.rs

@@ -30,6 +30,7 @@
 
 mod engine;
 pub mod mode_stack;
+pub mod mobile_voice_backend;
 pub mod ptt;
 pub mod ptt_backends;
 pub mod release_tail;
@@ -42,6 +43,9 @@ mod sdl_output;
 #[cfg(target_os = "ios")]
 mod ios_voice_unit;
 
+#[cfg(target_os = "android")]
+pub mod android_voice_unit;
+
 pub use engine::{AudioEngine, AudioEngineConfig};
 pub use ptt::{AudioTransmitGate, MissedKeyUpWatchdog, PttBackendDescriptor, PttCapabilityLevel};
 pub use ptt_backends::{