P4: lock bigkernel semantics (exp2(-mask)*blend) + calibrate scale

Read the 0x26b820 SIMD loop: the exp2 result is multiplied by the blend buffer
(vmulpd at 0x18026bba0 with ymm11=blend) => mask = exp2(-level)*blend, sign
inverted for attenuation. framed_model.cpp corrected to exp2(-scratch)*0.8.
Calibrated level_scale=600 -> t1kq err -0.06 dB (ref -14.98). comb per-tone
errors reduced (max ~7.8 dB); warp/FFT-conv smoothing still approximated.

dsp/framed_model.cpp

@@ -117,9 +117,12 @@ void FramedDetector::processFrame(const std::complex<double>* spectrum, float* m
 
         // 4. blend + bigkernel exp2 (0x26b820):
         //    b = freqaxis*(1-mix) + mix*0.8  (= 0.8 at mix=1.0)
-        //    mask = exp2(0.5 * (mask - b))   [exp2 kernel scales input by 0.5]
+        //    mask = exp2(-mask) * b          (vectorized exp2 of x blend;
+        //                                     verified from the SIMD loop: the
+        //                                     exp2 result is multiplied by the
+        //                                     blend buffer ymm11).
         for (size_t k = 0; k <= half; k++) {
-            scratch[k] = std::exp2(0.5 * (static_cast<double>(scratch[k]) - C_BLEND08));
+            scratch[k] = std::exp2(-static_cast<double>(scratch[k])) * C_BLEND08;
         }
 
         // 5. combine / accumulator (0x5407c8[band]):