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/phase_table.cpp

@@ -0,0 +1,11 @@
+#include "phase_table.hpp"
+#include <array>
+#include <cmath>
+
+const std::array<double, PHASE_TABLE_SIZE> phase_table = [] {
+    std::array<double, PHASE_TABLE_SIZE> table;
+    for (size_t k = 0; k < PHASE_TABLE_SIZE; k++) {
+        table[k] = std::sin(k * 2.0 * M_PI / PHASE_TABLE_SIZE);
+    }
+    return table;
+}();