P4: Phase B — framed_model re-transcribed to the confirmed chain (NOTES:199-226)

Fixes structural divergences: IIR1 into shared 0x5406f8 buffer + bridge to band
mask (FUN_18052d650, 0x5160); IIR2 on band mask; Hermitian mirror (0x11940);
blend step f6f8=axis(1-mix)+mix*0.8, mask=exp2(-mask)*f6f8; real combine via
kRTAtt/kRTRel weights (0x5406c8/6e8); warp, IIR3x2, dry/wet.

Validation: t1kq only1 fc1000 -21.1 vs -22.1 (OK); fc-scan shape intact;
dual @500 matches (s30 -51.5 vs -53.7), @2000 gap -4..-13 vs -29.6 remains.
That gap = dB-domain band LUT (FUN_180563a60) not yet in chain -> Phase A.

dsp/framed_model.hpp

@@ -45,5 +45,6 @@ private:
     std::vector<DetectorBand> bands_;
     std::vector<std::vector<float>> res_;   // per band, per bin |2B/A|
     std::vector<float> am_;                 // smoothed per-bin amplitude
+    std::vector<float> f6f8_;               // shared 0x5406f8 blend buffer (IIR1 out)
     std::vector<std::vector<float>> track_; // per band, per bin accumulator 0x5407c8
 };