feat(patina): per-band compressor gain stage with Threshold, Ratio and Knee

Tester feedback asked for a threshold and a ratio. Each band now cuts like a
small compressor: Threshold (dB above the local floor) replaces Selectivity,
Ratio scales the cut per dB above it (inf:1 is the old behaviour), and a
Soft/Hard knee switch sets the curve. A small graph shows bump height in, cut
out. Mix and Output move to the slider panel so the detection panel fits.
common-ui gains rotary_text for knobs with custom value text.

plugins/patina/core/tests/dsp.rs

@@ -148,7 +148,7 @@ fn resonant_bump_is_cut_where_it_is_and_nowhere_else() {
     }
 }
 #[test]
-fn depth_selectivity_and_focus_each_do_one_thing() {
+fn depth_threshold_and_focus_each_do_one_thing() {
     let run = |s: Settings| {
         let mut bump = Peak::new(3000., 4., 12., 48000.);
         let frame = settle(s, |i| bump.tick(saw(220., i, 48000.)));
@@ -160,7 +160,7 @@ fn depth_selectivity_and_focus_each_do_one_thing() {
         ..Default::default()
     });
     let picky = run(Settings {
-        selectivity: 100.,
+        threshold: 5.5,
         ..Default::default()
     });
     let mut protect = Settings::default();
@@ -279,7 +279,7 @@ fn invalid_settings_do_not_mutate_and_numerical_inputs_stay_finite() {
 }
 #[test]
 fn soft_knee_and_presets_are_valid() {
-    for i in 0..4 {
+    for i in 0..Settings::PRESETS.len() {
         Settings::preset(i).validate().unwrap();
     }
     assert_eq!(knee(-2., 3.), 0.);
@@ -469,3 +469,103 @@ fn old_settings_without_tone_guard_still_load() {
     let s: Settings = serde_json::from_value(json).unwrap();
     assert_eq!(s.tone_guard, 0.);
 }
+
+/// Cut at the bump for a resonance of `gain` dB (Q 4 at 3 kHz) on a 220 Hz saw.
+fn bump_cut(s: Settings, gain: f32) -> f32 {
+    let mut bump = Peak::new(3000., 4., gain, 48000.);
+    let frame = settle(
+        Settings {
+            transient_guard: 0.,
+            ..s
+        },
+        |i| bump.tick(saw(220., i, 48000.)),
+    );
+    peak_reduction(&frame, 3000. / 1.26, 3000. * 1.26)
+}
+#[test]
+fn ratio_spares_small_peaks_and_still_cuts_big_ones() {
+    let at = |ratio: f32, gain: f32| {
+        bump_cut(
+            Settings {
+                depth: 20.,
+                ratio,
+                ..Default::default()
+            },
+            gain,
+        )
+    };
+    let (small_hard, big_hard) = (at(RATIO_INFINITE, 6.), at(RATIO_INFINITE, 18.));
+    let (small, big) = (at(2., 6.), at(2., 18.));
+    // At 2:1 each cut is about half the ∞:1 cut.
+    for (half, full) in [(small, small_hard), (big, big_hard)] {
+        assert!((half / full - 0.5).abs() < 0.15, "{half} vs {full}");
+    }
+    assert!(small < 1., "small bump cut by {small} dB at 2:1");
+    assert!(big > 4., "big bump cut only {big} dB at 2:1");
+    // Ratio between the two ends gives a cut between them.
+    let mid = at(4., 18.);
+    assert!(big < mid && mid < big_hard, "{big} < {mid} < {big_hard}");
+}
+#[test]
+fn cut_grows_with_the_bump_until_depth() {
+    let s = Settings {
+        depth: 20.,
+        ratio: 3.,
+        ..Default::default()
+    };
+    let cuts: Vec<f32> = [4., 8., 12., 18., 24.]
+        .iter()
+        .map(|g| bump_cut(s, *g))
+        .collect();
+    for w in cuts.windows(2) {
+        assert!(w[1] > w[0] + 0.5, "{cuts:?}");
+    }
+    let capped = bump_cut(Settings { depth: 4., ..s }, 24.);
+    assert!(capped <= 4.001 && capped > 3., "{capped}");
+}
+#[test]
+fn threshold_moves_where_cuts_start() {
+    let low = bump_cut(Settings::default(), 9.);
+    let high = bump_cut(
+        Settings {
+            threshold: 9.,
+            ..Default::default()
+        },
+        9.,
+    );
+    assert!(low > 2. && high < 0.5, "{low} / {high}");
+}
+#[test]
+fn hard_knee_starts_cutting_closer_to_the_threshold() {
+    // A bump just under the threshold: the soft knee already cuts a little, the hard knee not.
+    let s = Settings {
+        threshold: 4.,
+        ..Default::default()
+    };
+    let soft = bump_cut(s, 6.);
+    let hard = bump_cut(
+        Settings {
+            knee: Knee::Hard,
+            ..s
+        },
+        6.,
+    );
+    assert!(soft > hard + 0.2, "soft {soft} vs hard {hard}");
+    assert_eq!(knee(0.25, 1.), 0.28125);
+}
+#[test]
+fn settings_from_before_threshold_still_load() {
+    let mut json = serde_json::to_value(Settings::default()).unwrap();
+    let map = json.as_object_mut().unwrap();
+    for key in ["threshold", "ratio", "knee", "tone_guard"] {
+        map.remove(key);
+    }
+    map.insert("selectivity".into(), 100.into());
+    let s: Settings = serde_json::from_value(json).unwrap();
+    assert_eq!(s.threshold, 5.5);
+    assert_eq!(s.ratio, RATIO_INFINITE);
+    assert_eq!(s.knee, Knee::Soft);
+    assert!(serde_json::from_str::<Settings>(r#"{"oops":1}"#).is_err());
+    let back: Settings = serde_json::from_str(&serde_json::to_string(&s).unwrap()).unwrap();
+    assert_eq!(back, s);
+}