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/ms.hpp

@@ -0,0 +1,31 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+
+inline void encode_ms(float* left, float* right, size_t n) {
+    for (size_t i = 0; i < n; i++) {
+        float mid = (left[i] + right[i]) * 0.5f;
+        float side = (left[i] - right[i]) * 0.5f;
+        left[i] = mid;
+        right[i] = side;
+    }
+}
+
+inline void decode_ms(float* left, float* right, size_t n) {
+    for (size_t i = 0; i < n; i++) {
+        float mid = left[i];
+        float side = right[i];
+        left[i] = mid + side;
+        right[i] = mid - side;
+    }
+}
+
+inline void apply_balance(float* left, float* right, float balance, size_t n) {
+    float gain_l = std::sqrt(0.5f * (1 - balance));
+    float gain_r = std::sqrt(0.5f * (1 + balance));
+    for (size_t i = 0; i < n; i++) {
+        left[i] *= gain_l;
+        right[i] *= gain_r;
+    }
+}
+