[fix](trx-backend-soapysdr): enforce 9 kHz minimum bandwidth for AMC mode

clamp_bandwidth_for_mode now floors AMC bandwidth at 9 kHz so that both
the sum (L+R) and difference (L−R) sidebands are always captured,
regardless of what the user or a set_filter call requests.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/trx-backend/trx-backend-soapysdr/src/dsp/channel.rs

@@ -217,8 +217,12 @@ pub struct ChannelDsp {
 
 impl ChannelDsp {
     fn clamp_bandwidth_for_mode(mode: &RigMode, bandwidth_hz: u32) -> u32 {
-        let _ = mode;
-        bandwidth_hz
+        match mode {
+            // C-QUAM requires ≥ 9 kHz to capture both sum (L+R) and difference
+            // (L−R) sidebands; narrower bandwidths would discard stereo content.
+            RigMode::AMC => bandwidth_hz.max(9_000),
+            _ => bandwidth_hz,
+        }
     }
 
     pub fn set_channel_if_hz(&mut self, channel_if_hz: f64) {