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/fft.hpp

@@ -0,0 +1,14 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <complex>
+#include "fft_plan.hpp"
+
+namespace fft {
+
+void init_plan(FFTPlan* plan, uint32_t log2N);
+void build_twiddle(FFTPlan* plan, double* scratch);
+void execute(const FFTPlan* plan, std::complex<double>* buf);
+void execute_inverse(const FFTPlan* plan, std::complex<double>* buf);
+
+}