P1.3: integrate FFT-conv stage (build_fir_from_window step5 memcpy + fir_from_mask + overlap-save conv) with captured WIN_WINDOW; fftconv_check confirms FIR=window[2048:4096]={0.8->1.0}

dsp/fftconv.hpp

@@ -0,0 +1,35 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+#include <complex>
+#include "fft_plan.hpp"
+
+// Real-time FFT-convolution stage mirroring the plugin per-band FFT-conv
+// (NOTES_LEVEL 2026-08-19d, FUN_18052b550 steps 3-6):
+//   FIR built from the captured WIN_WINDOW tail (step 5 memcpy),
+//   then overlap-save convolution applied to the audio.
+// Structural transcription; gain accuracy depends on the mask formed upstream
+// (level LUT + twin resonance), which is a separate stage (P2+).
+namespace fftconv {
+
+// Builds the per-band FIR from the captured window: copies window[N/2..N-1]
+// into FIR[0..N/2-1] and fills the upper half with zero (xmm9 fill), N = nfft.
+// Mirrors the decompiled step 5 exactly.
+void build_fir_from_window(double* fir, const float* window, size_t nfft);
+
+// FIR impulse response from an arbitrary spectrum buffer (input `spec` of
+// nfft/2+1 complex doubles) via forward FFT + step-5 window blend + inverse.
+void fir_from_mask(std::complex<double>* fir,
+                   const std::complex<double>* mask,
+                   const float* window,
+                   size_t nfft,
+                   const FFTPlan* plan);
+
+// Overlap-save convolution of `in` (frames) with real FIR `ir` (nfft samples).
+// out pre-sized to frames. lat: zero-pad/initial delay applied internally.
+void conv_overlap_save(const double* ir, size_t nfft, size_t hop,
+                       const float* in, float* out, size_t frames,
+                       const FFTPlan* plan);
+
+}  // namespace fftconv