[fix](trx-backend-soapysdr): improve wfm stereo blend threshold

Raise the stereo blend floor from 0.55 to 0.75 at pilot lock and lower
the full-blend ceiling from stereo_detect_level 0.92 to 0.70.  This
gives real-world signals with moderate pilot strength much better L/R
separation (~17 dB immediately at lock vs ~5 dB before) and reaches
full unity blend sooner.

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

@@ -689,8 +689,8 @@ impl WfmStereoDecoder {
                 self.stereo_detected = true;
             }
             let stereo_blend_target = if self.stereo_detected {
-                let width = smoothstep01((self.stereo_detect_level - 0.30) / (0.92 - 0.30));
-                0.55 + 0.45 * width
+                let width = smoothstep01((self.stereo_detect_level - 0.30) / (0.70 - 0.30));
+                0.75 + 0.25 * width
             } else {
                 0.35 * smoothstep01((self.stereo_detect_level - 0.10) / (0.30 - 0.10))
             };