feat(bridge,core): add is_hard_muted and get_output_gain readback (TODO-037)

Add bridge read-back functions for hard-mute state and output gain.
Add core output_gain() getter. Document feature gaps in SRS.

core/chanora_core/src/lib.rs

@@ -1113,6 +1113,21 @@ impl ChanoraSession {
         Ok(())
     }
 
+    /// Read the current master output gain. Returns `1.0` (unity) if
+    /// audio is not started.
+    pub async fn output_gain(&self) -> f32 {
+        let guard = self.inner.lock().await;
+        let state = match guard.as_ref() {
+            Some(s) => s,
+            None => return 1.0,
+        };
+        let audio = match state.audio.as_ref() {
+            Some(a) => a,
+            None => return 1.0,
+        };
+        audio.output_gain()
+    }
+
     /// Set per-client output volume (SRS-075). `1.0` is unity, `0.0`
     /// mutes. No-op if audio is not started or client has no active
     /// voice queue.