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/filter.hpp

@@ -0,0 +1,42 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+constexpr size_t MAX_BANDS = 8;
+
+struct FilterParams {
+    float freq;
+    float q;
+    float gain;
+    int type;
+    int on;
+};
+
+struct FilterState {
+    float x1, x2, y1, y2;
+};
+
+class DigitalFilter {
+public:
+    DigitalFilter();
+    void setParams(const FilterParams& p, float sample_rate);
+    float process(float x);
+    
+private:
+    FilterParams params_;
+    FilterState state_;
+    float b0, b1, b2, a1, a2;
+    float fs_;
+    
+    void updateCoeffs();
+};
+
+class FilterGraph {
+public:
+    FilterGraph();
+    void processBlock(float* in, float* out, size_t n, const FilterParams* bands, size_t num_bands);
+    
+private:
+    DigitalFilter filters_[MAX_BANDS];
+};