Add --cut-slope option for filter steepness (default 12 dB/oct); cascade biquads for steeper slopes; update docs

pkg/convolve/convolve.go

@@ -425,3 +425,31 @@ func ApplyHighpassButterworth(data []float64, sampleRate int, cutoffHz float64)
 	}
 	return out
 }
+
+// CascadeLowcut applies the low-cut (high-pass) filter multiple times for steeper slopes.
+// slopeDb: 12, 24, 36, ... (dB/octave)
+func CascadeLowcut(data []float64, sampleRate int, cutoffHz float64, slopeDb int) []float64 {
+	if slopeDb < 12 {
+		slopeDb = 12
+	}
+	n := slopeDb / 12
+	out := data
+	for i := 0; i < n; i++ {
+		out = ApplyHighpassButterworth(out, sampleRate, cutoffHz)
+	}
+	return out
+}
+
+// CascadeHighcut applies the high-cut (low-pass) filter multiple times for steeper slopes.
+// slopeDb: 12, 24, 36, ... (dB/octave)
+func CascadeHighcut(data []float64, sampleRate int, cutoffHz float64, slopeDb int) []float64 {
+	if slopeDb < 12 {
+		slopeDb = 12
+	}
+	n := slopeDb / 12
+	out := data
+	for i := 0; i < n; i++ {
+		out = ApplyLowpassButterworth(out, sampleRate, cutoffHz)
+	}
+	return out
+}