[feat](trx-core): add AMC variant to RigMode for C-QUAM AM stereo

Add RigMode::AMC to represent Motorola C-QUAM Compatible Quadrature
Amplitude Modulation stereo, placing it after RigMode::AM.

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

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

@@ -85,6 +85,8 @@ pub enum RigMode {
     CW,
     CWR,
     AM,
+    /// AM C-QUAM stereo (Compatible Quadrature Amplitude Modulation).
+    AMC,
     WFM,
     FM,
     AIS,