refactor(audio): remove experimental iOS RemoteIO+WebRTC APM path

Delete ios_raw_unit.rs (538 lines) and all SonoraExperimental references
from the core audio crate. The experimental RemoteIO path that bypassed
Apple VPIO in favor of software WebRTC APM was never shipped and is no
longer needed. VPIO is the sole production iOS audio backend.

- Delete ios_raw_unit.rs entirely
- Remove Raw variant from IosVoiceBackend enum in engine.rs
- Remove SonoraExperimental from IosVoiceProcessingMode enum
- Simplify validate_for_ios() (single-variant enum, no mode check)
- Remove 2 SonoraExperimental validation tests
- Remove ios_raw_unit module declaration from lib.rs
- Remove include_str!-based debug_wav test for the deleted file

crates/chanora_audio/src/lib.rs

@@ -65,10 +65,7 @@ pub(crate) mod voice_render;
 mod sdl_output;
 
 #[cfg(any(target_os = "ios", target_os = "macos"))]
-mod ios_voice_unit;
-
-#[cfg(target_os = "ios")]
-pub mod ios_raw_unit;
+ mod ios_voice_unit;
 
 #[cfg(target_os = "android")]
 pub mod android_voice_unit;