[feat](trx-core): add AudioSource trait and RigCat::as_audio_source

Add AudioSource trait to trx-core rig module providing subscribe_pcm()
for demodulated PCM audio. Add opt-in as_audio_source() default method
to RigCat returning None; SDR backends will override to return Some(self).
Re-export AudioSource from the crate root. Marks SDR-01 complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-core/src/lib.rs

@@ -10,6 +10,7 @@ pub mod rig;
 
 pub type DynResult<T> = Result<T, Box<dyn std::error::Error + Send + Sync>>;
 
+pub use rig::AudioSource;
 pub use rig::command::RigCommand;
 pub use rig::request::RigRequest;
 pub use rig::response::{RigError, RigResult};

src/trx-core/src/rig/mod.rs

@@ -57,6 +57,13 @@ fn default_min_freq_step_hz() -> u64 {
     1
 }
 
+/// Trait for rigs that can provide demodulated PCM audio.
+pub trait AudioSource: Send + Sync {
+    /// Subscribe to demodulated PCM audio from the primary channel.
+    /// Returns a broadcast receiver that yields 20ms frames of mono f32 PCM.
+    fn subscribe_pcm(&self) -> tokio::sync::broadcast::Receiver<Vec<f32>>;
+}
+
 /// Common interface for rig backends.
 pub trait Rig {
     fn info(&self) -> &RigInfo;
@@ -103,6 +110,8 @@ pub trait RigCat: Rig + Send {
     fn lock<'a>(&'a mut self) -> Pin<Box<dyn Future<Output = DynResult<()>> + Send + 'a>>;
 
     fn unlock<'a>(&'a mut self) -> Pin<Box<dyn Future<Output = DynResult<()>> + Send + 'a>>;
+
+    fn as_audio_source(&self) -> Option<&dyn AudioSource> { None }
 }
 
 /// Snapshot of a rig's status that every backend can expose.