dsp/: C++ skeleton with working FFT/WOLA STFT

- Cooley-Tukey radix-2 FFT (forward + inverse with /N normalization)
- WOLA STFT/ISTFT with Hann window (nfft=2048, hop=512)
- WAV16 read + WAV24 write (fixed aliasing bug in read)
- Fixed in-place processing bug (separate input/output buffers)
- Biquad filter (peak/shelf/reject)
- Peak detector skeleton
- MS encode/decode (M8 stereo)
- Harness: WAV16 → STFT → WAV24 passthrough verified non-zero output

Verified: burst500.wav passthrough produces output RMS=0.0678

dsp/spectral.hpp

@@ -0,0 +1,34 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <complex>
+#include <vector>
+#include "fft.hpp"
+
+constexpr size_t DEFAULT_NFFT = 2048;
+constexpr size_t DEFAULT_HOP = 512;
+
+class SpectralProcessor {
+public:
+    SpectralProcessor(size_t nfft = DEFAULT_NFFT, size_t hop = DEFAULT_HOP);
+    ~SpectralProcessor();
+    
+    void processBlock(float* in, float* out, size_t num_samples, size_t num_channels = 1);
+    
+private:
+    size_t nfft_;
+    size_t hop_;
+    double* window_;
+    FFTPlan plan_;
+    std::complex<double>* buf_;
+    std::complex<double>* tmp_buf_;
+    std::vector<float> overlap_;
+    size_t frame_count_;
+    size_t output_pos_;
+    
+    void computeWindow();
+    void stftFrame(const float* in, std::complex<double>* out);
+    void istftFrame(std::complex<double>* in, float* out, float* overlap);
+    void updateDetector();
+};
+