feat: consumer identified (th_b3c0), scan3.py, RT_FIRCONV/RT_FIRPOWER

- th_b3c0 (0x18000b3c0) = pure complex multiply FIR × audio in freq-domain
- scan3.py: pre-scan approach finds ctx in 1.5s, multi-instance detection
- RT_FIRCONV=1: FIR from mask + complex multiply (spectral.cpp)
- RT_FIRPOWER=1: power-law mask from raw spectrum (framed_model.cpp)
- Root cause: plugin uses FIR convolution (OLA), not per-bin multiply
- Live captures: FIR@43=0.524, mask@43=0.510, final gain=0.305
- Best result: RT_LUT_OFF gives cut@500=-8.18 dB (ref -10.32)
- NOTES_LEVEL 24e/24f/24g appended

dsp/spectral.hpp

@@ -25,6 +25,9 @@ private:
     FFTPlan plan_;
     std::complex<double>* buf_;
     std::complex<double>* tmp_buf_;
+    std::complex<double>* fir_buf_;
+    std::complex<double>* fir_freq_;
+    std::vector<double> fir_window_;
     std::vector<float> overlap_;
     std::vector<float> mask_;
     FramedDetector detector_;