dsp/: add spectral detector with envelope + peak suppression

- Detector computes smoothed spectral envelope
- Finds peaks exceeding envelope
- Creates per-bin suppression mask
- Applies mask in frequency domain before ISTFT
- Default params: sharpness=1.0, selectivity=0.5, depth=0.3

Verified: burst500.wav → output RMS reduced from 0.0678 to 0.0476

dsp/harness.cpp

@@ -131,6 +131,7 @@ int main(int argc, char* argv[]) {
     std::vector<float> output(total_samples);
     
     SpectralProcessor sp(2048, 512);
+    sp.setDetectorParams(1.0f, 0.5f, 0.3f);
     
     std::vector<float> left_in(frames), right_in(frames);
     for (size_t i = 0; i < frames; i++) {