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.cpp

@@ -0,0 +1,30 @@
+#include "twiddle_builder.hpp"
+#include <cmath>
+#include <cstring>
+
+namespace twiddle {
+
+double* build_twiddle(uint32_t log2N, double* dst) {
+    uint32_t N = 1U << log2N;
+    uint32_t N_quarter = N / 4;
+    
+    if (log2N < 11) {
+        uint32_t stride = 1U << (10 - log2N);
+        for (uint32_t k = 0; k < N_quarter; k++) {
+            dst[k] = phase_table[k * stride];
+        }
+    } else {
+        double angle_step = (2.0 * M_PI) / N;
+        for (uint32_t k = 0; k < N_quarter; k++) {
+            dst[k] = k * angle_step;
+        }
+    }
+    
+    dst[N_quarter] = 1.0;
+    
+    uint8_t* p = reinterpret_cast<uint8_t*>(dst);
+    uintptr_t aligned = (reinterpret_cast<uintptr_t>(p + N * 8 + 0x3f) & ~0x3f);
+    return reinterpret_cast<double*>(aligned);
+}
+
+}