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.

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.