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.cpp

@@ -0,0 +1,37 @@
+#include "fft_plan.hpp"
+#include <cstring>
+
+namespace fft {
+
+void init_plan(FFTPlan* plan, uint32_t log2N) {
+    plan->log2N = log2N;
+    plan->N = 1U << log2N;
+    plan->stage_count = 0;
+    plan->bit_reverse = 0;
+    plan->xor_mask = 0;
+    
+    uint32_t n = plan->N;
+    while (n > 1) {
+        if (n % 8 == 0 && log2N >= 3) {
+            plan->stages[plan->stage_count].radix = 8;
+            plan->stages[plan->stage_count].group_size = n / 8;
+            plan->stages[plan->stage_count].groups = 8;
+            plan->stage_count++;
+            n /= 8;
+        } else if (n % 4 == 0 && log2N >= 2) {
+            plan->stages[plan->stage_count].radix = 4;
+            plan->stages[plan->stage_count].group_size = n / 4;
+            plan->stages[plan->stage_count].groups = 4;
+            plan->stage_count++;
+            n /= 4;
+        } else if (n % 2 == 0) {
+            plan->stages[plan->stage_count].radix = 2;
+            plan->stages[plan->stage_count].group_size = n / 2;
+            plan->stages[plan->stage_count].groups = 2;
+            plan->stage_count++;
+            n /= 2;
+        }
+    }
+}
+
+}