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_plan.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+
+struct FFTStage {
+    uint32_t radix;      // 2, 4, или 8
+    uint32_t group_size; // N / radix
+    uint32_t groups;     // количество групп
+};
+
+struct FFTPlan {
+    uint32_t log2N;
+    uint32_t N;
+    uint32_t stage_count;
+    FFTStage stages[16];
+    uint32_t bit_reverse;   // 1 если нужна бит-реверсия
+    uint64_t xor_mask;      // маска для XOR при бит-реверсии (DAT_181c5e4e0)
+};
+
+namespace fft {
+
+void init_plan(FFTPlan* plan, uint32_t log2N);
+
+}