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/detect.hpp

@@ -2,26 +2,22 @@
 #include <cstddef>
 #include <cstdint>
 #include <complex>
-
-struct Peak {
-    size_t bin;
-    float freq;
-    float magnitude;
-    float reduction;
-};
+#include <vector>
 
 class Detector {
 public:
-    Detector();
+    Detector(size_t nfft, float sample_rate);
     void setParams(float sharpness, float selectivity, float depth);
-    size_t detectPeaks(const std::complex<double>* spectrum, size_t n,
-                      float sample_rate, Peak* peaks, size_t max_peaks);
-    
+    void processFrame(const std::complex<double>* spectrum, float* mask);
+
 private:
+    size_t nfft_;
+    float sample_rate_;
     float sharpness_;
     float selectivity_;
     float depth_;
-    
-    float computeReduction(float magnitude, float freq);
-};
 
+    std::vector<float> envelope_;
+    std::vector<float> prev_mask_;
+    std::vector<float> smooth_buf_;
+};