[fix](trx-backend-soapysdr): increase wfm stereo matrix gain to 0.80

More stereo headroom for broadcast audio. With WFM_OUTPUT_GAIN at 0.35
the effective output is 0.28 peak, well within clipping margin.

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

src/trx-server/trx-backend/trx-backend-soapysdr/src/demod.rs

@@ -33,7 +33,7 @@ const STEREO_SEPARATION_PHASE_TRIM: f32 = 0.0;
 const STEREO_SEPARATION_GAIN: f32 = 1.000;
 /// Extra headroom in the stereo matrix to reduce stereo-only clipping/IMD on
 /// strong program material. This keeps bass excursions from flattening treble.
-const STEREO_MATRIX_GAIN: f32 = 0.50;
+const STEREO_MATRIX_GAIN: f32 = 0.80;
 /// Stereo detection runs every N composite samples to reduce CPU.
 const STEREO_DETECT_DECIMATION: u32 = 16;
 /// Gentle high-pass memory for the stereo L-R path.