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_stage.cpp

@@ -0,0 +1,40 @@
+#include "fft_stage.hpp"
+#include <cstring>
+
+namespace fft_stage {
+
+void cplx_mul(double* out, const double* in1, const double* in2, uint32_t n) {
+    for (uint32_t i = 0; i < n; i += 64) {
+        for (uint32_t j = 0; j < 4; j++) {
+            double re1 = in1[i + j*2 + 0];
+            double im1 = in1[i + j*2 + 1];
+            double re2 = in2[i + j*2 + 0];
+            double im2 = in2[i + j*2 + 1];
+            out[i + j*2 + 0] = re1*re2 - im1*im2;
+            out[i + j*2 + 1] = re1*im2 + im1*re2;
+        }
+    }
+}
+
+void stage_complex(double* out, const double* in, const double* tw, uint32_t n) {
+    for (uint32_t i = 0; i < n; i += 64) {
+        for (uint32_t j = 0; j < 4; j++) {
+            double re1 = in[i + j*2 + 0];
+            double im1 = in[i + j*2 + 1];
+            double re2 = tw[i + j*2 + 0];
+            double im2 = tw[i + j*2 + 1];
+            out[i + j*2 + 0] = re1*re2 - im1*im2;
+            out[i + j*2 + 1] = re1*im2 + im1*re2;
+        }
+    }
+}
+
+void stage_double(double* out, const double* in, const double* tw, uint32_t n) {
+    for (uint32_t i = 0; i < n; i += 64) {
+        for (uint32_t j = 0; j < 8; j++) {
+            out[i + j] = in[i + j] * tw[i + j];
+        }
+    }
+}
+
+}