STFT partitioned conv: FIR construction pipeline (52b550-52b8bb)

Implement minimum-phase FIR design from BLOCKMAP:
- buildFirFromMask: mask → 1/mask (reciprocal via log→negate→exp) →
  IFFT → causal window → FFT → normalize → complex multiply
- RT_FIRCONV=2 activates the new path
- RT_FIRCONV=1 preserved as simple mask × audio (legacy)

Results (tone1kq single band):
  default (pointwise):  500Hz=-25.35 dB, 1kHz=-50.60 dB
  FIRCONV=1 (mask mul): 500Hz=-1.31 dB, 1kHz=-25.92 dB
  FIRCONV=2 (min-phase): same as FIRCONV=1

The twiddle stages (ops B/C/D with cos/sin tables) are the missing
piece for bit-exact FIR construction. They perform FMA operations
with twiddle factors that modify the mask shape.

Note: dual_b1q_0.5.wav reference is empty (0 bytes) — corpus can't run.
Needs regeneration.

dsp/spectral.hpp

@@ -37,4 +37,14 @@ private:
     void computeWindow();
     void stftFrame(const float* in, std::complex<double>* out);
     void istftFrame(std::complex<double>* in, float* out, float* overlap);
+
+    // FIR construction from detector mask (52b550-52b8bb pipeline):
+    // mask → log → sign-invert → EXP → twiddle ops → window → normalize
+    // Produces frequency-domain FIR kernel for complex multiply application.
+    void buildFirFromMask(const float* mask, std::complex<double>* fir, size_t nbin);
+
+    // WIN_freq: live-captured freq-path window (0x540658), 0.5→1.0
+    std::vector<float> win_freq_;
+    bool win_freq_loaded_ = false;
+    void loadWinFreq();
 };