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/twiddle_builder.hpp

@@ -0,0 +1,11 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cmath>
+#include "phase_table.hpp"
+
+namespace twiddle {
+
+double* build_twiddle(uint32_t log2N, double* dst);
+
+}