feat(patina): Tone guard for bass lines

Patina detects the played note (30-500 Hz) and remembers, over about
4 s, how high each spectral bump is at a fixed frequency and relative
to the note. Bumps that follow the note (a boosted harmonic the patch
is built on) get less cut; bumps fixed in Hz (formants, room modes)
are cut as before. On a saw line F1-F2 with harmonic 6 boosted, the
cut on that harmonic drops from 5.78 dB to 0.44 dB at 100 %, while a
fixed formant at 1.5 kHz stays at 5.78 dB. One held note is cut as
normal, since the two cases cannot be told apart.

- New parameter tone-guard (0-100 %, default 0), appended at the end;
  0 % is the old behaviour and old settings load unchanged.
- New Bass line preset.
- Editor: Tone guard control, NOTE readout and a strip over the
  spectrum where cuts are held back.
- No added latency, no allocation on the audio thread; patina-core now
  depends on common-dsp.

plugins/patina/plugin/src/params.rs

@@ -62,6 +62,8 @@ pub struct PatinaParams {
     pub mix: FloatParam,
     #[id = "output"]
     pub output: FloatParam,
+    #[id = "tone-guard"]
+    pub tone_guard: FloatParam,
     #[id = "bypass"]
     pub bypass: BoolParam,
     #[nested(array, group = "Focus")]
@@ -82,6 +84,7 @@ impl Default for PatinaParams {
             high: log("High frequency", d.high, 21., 20000., " Hz"),
             mix: linear("Mix", 100., 0., 100., " %"),
             output: linear("Output", 0., -12., 12., " dB"),
+            tone_guard: linear("Tone guard", d.tone_guard, 0., 100., " %"),
             bypass: BoolParam::new("Bypass", false).make_bypass(),
             focus: std::array::from_fn(|_| FocusParams::default()),
         }
@@ -116,6 +119,7 @@ impl PatinaParams {
             &self.focus[3].frequency,
             &self.focus[3].width,
             &self.focus[3].sensitivity,
+            &self.tone_guard,
         ]
     }
     pub fn settings(&self) -> Settings {
@@ -137,6 +141,7 @@ impl PatinaParams {
                 width: self.focus[i].width.value(),
                 sensitivity: self.focus[i].sensitivity.value(),
             }),
+            tone_guard: self.tone_guard.value(),
         };
         // Independent automation of bounds may cross them; preserve a valid effective range.
         if s.low >= s.high {
@@ -145,8 +150,8 @@ impl PatinaParams {
         s
     }
 }
-/// Host float parameters: ten main controls, then four per focus point.
-pub const COUNT: usize = 26;
+/// Host float parameters: ten main controls, four per focus point, then Tone guard.
+pub const COUNT: usize = 27;
 pub fn values(s: &Settings) -> [f32; COUNT] {
     let mut v = [0.; COUNT];
     v[..10].copy_from_slice(&[
@@ -169,5 +174,6 @@ pub fn values(s: &Settings) -> [f32; COUNT] {
             s.focus[i].sensitivity,
         ]);
     }
+    v[26] = s.tone_guard;
     v
 }