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]):

handoff/NOTES_LEVEL.md

@@ -850,3 +850,17 @@ Read /tmp/consumers_out.txt:471-1277 (full decomp) + f529fe0.dis + live snap_rt.
 
 ### NOTE (SR mismatch): internal DSP SR=48000, N=4096 (bin spacing 11.713 Hz). Host refs 44100.
  IIR tables indexed 0..2048 (N/2+1). For bit-exact the detector must run N=4096@48k internally.
+
+## ============ UPDATE 2026-08-20k: bigkernel 0x26b820 = vectorized exp2 (semantics LOCKED) ============
+Read the SIMD loop (objdump 0x18026b820-0x18026bcaf). Key line 0x18026bba0:
+  vmulpd ymm12 (exp2 result) by ymm11 (loaded from rbp = blend buffer 0x5406f8).
+=> mask[i] = exp2(x) * blend[i], where x is the mask value (0x540678[band]) and
+   blend = freqaxis*(1-mix)+mix*0.8 (=0.8 at mix=1.0). sign: to get attenuation
+   (mask<1) x must be negative, so mask = exp2(-level) * blend. Locked.
+exp2 is standard log2e/poly/mantissa-table (0x1f34a80..., log2e~1.4424, floor -708.9).
+
+### Calibration (framed_test, structural chain now in framed_model.cpp):
+- t1kq single band (678.76,q~1,sens12): level_scale=600 -> err -0.06 dB (ref -14.98).
+- comb 4-band at scale 600: per-tone err 300:-5.1, 500:+7.8, 1000:+4.4, 1500:+3.9,
+  2000:-1.3, 3000:+4.8 dB (max 7.8). Mask shape (warp tilt + FFT-conv smoothing)
+  still approximated.