feat: stable note tracking, NOTE pitch source and unsure fallback

Producers found the tracked plugins jumpy: the shared tracker switched notes
after 10 ms and retuned every 5 ms, so growls and formant basses flickered
and every tracked plugin acted on wrong notes.

Shared tracker (common-dsp):
- A new note needs a steady read for 40 ms; an octave down 80 ms, an octave
  up 200 ms (an octave-up read still fits the note's period and counts as
  agreeing). Inside a note the tuning only moves past a 0.25 semitone
  deadband. Follow bends keeps the old fast behaviour as an option.
- A trust score turns the note unsure while reads disagree; Tracker::active
  and NoteState report it, and Fallback latches "step back" across note ends.
- Notes end when the window start stays below the gate's release level for
  20 ms, so short gaps between notes are seen without waiting for the gate.
- PitchSource replaces the old (midi, gate, glide) arguments: Audio, MIDI or
  Note mode (a set note, no detection), plus a key lock (minor, major, root
  only) that snaps reads to the key.
- stems.rs: synthetic stems (growls, wobble, reese, formant, 808 slide,
  layered drop, noise riser) and a score; examples/tracker_report scores the
  corpus or real WAV stems. On the corpus the old behaviour makes up to 28
  extra note starts per second; the stable default makes none.

Plugins:
- Anvil, Lathe, Temper, Knurl, Planish, Braze and Forge gain the NOTE pitch
  source, the key lock and Follow bends (shared common-params crate and a
  shared header switch in common-ui).
- Unsure tracking: Anvil brings the original sub back in place of the sine,
  Lathe passes the audio through, Knurl fades its texture out; Temper,
  Planish and Braze hold their last sure tuning. Readouts show the note only
  when it is sure.
- Anvil gains Mode: Replace or Enhance (keeps the original sub and adds the
  sine under it).
- Planish slides its band edges over 6 ms on a new note instead of snapping.
- Ballast defaults to Grid parts, which need no tracking.

plugins/braze/core/src/lib.rs

@@ -181,7 +181,9 @@ impl Engine {
     }
     pub fn set_settings(&mut self, s: Settings) -> Result<(), &'static str> {
         s.validate()?;
-        self.tracker.set(s.source == Source::Midi, s.gate, true);
+        // Unsure tracking sends no events, so the beating holds its last sure note.
+        self.tracker
+            .set(s.pitch.source(s.source.mode(), s.gate, true));
         if s.low_cut != self.settings.low_cut || self.fresh {
             let q = std::f32::consts::FRAC_1_SQRT_2;
             let hz = s.low_cut.min(0.45 * self.rate);
@@ -340,7 +342,7 @@ impl Engine {
             let beat_hz = self.tempo / 60. / s.rate.quarters();
             f.ratio = 1. + (beat_hz / (s.harmonic as f64 * hz as f64)) as f32;
         }
-        if self.tracker.sounding() && hz > 0. {
+        if self.tracker.active() && hz > 0. {
             f.note = Some(hz_to_midi(hz).round().clamp(0., 127.) as u8);
         }
         f

plugins/braze/core/src/settings.rs

@@ -1,4 +1,5 @@
 //! Host-independent controls. Levels in dB, amounts in percent, angles in degrees.
+use common_dsp::tracker::{Pitch, PitchMode};
 use serde::{Deserialize, Serialize};
 
 /// Highest harmonic the beat rate can be set on.
@@ -14,6 +15,24 @@ pub enum Source {
     Audio,
     /// Notes come from MIDI note input.
     Midi,
+    /// Note starts are detected, the pitch is the set note: nothing is guessed.
+    Note,
+}
+impl Source {
+    pub fn mode(self) -> PitchMode {
+        match self {
+            Source::Audio => PitchMode::Audio,
+            Source::Midi => PitchMode::Midi,
+            Source::Note => PitchMode::Note,
+        }
+    }
+    pub fn from_mode(mode: PitchMode) -> Self {
+        match mode {
+            PitchMode::Audio => Source::Audio,
+            PitchMode::Midi => Source::Midi,
+            PitchMode::Note => Source::Note,
+        }
+    }
 }
 
 /// One beat cycle lasts this note value.
@@ -75,6 +94,9 @@ impl Rate {
 #[serde(deny_unknown_fields)]
 pub struct Settings {
     pub source: Source,
+    /// Note mode's note, the key lock and Follow bends.
+    #[serde(default)]
+    pub pitch: Pitch,
     /// How long one beat cycle of `harmonic` lasts.
     pub rate: Rate,
     /// The harmonic that beats exactly at `rate`. Harmonic n beats n/harmonic times as fast.
@@ -100,6 +122,7 @@ impl Default for Settings {
     fn default() -> Self {
         Self {
             source: Source::Audio,
+            pitch: Pitch::default(),
             rate: Rate::Eighth,
             // Harmonic 4 sits above the low cut on every bass note, and on F1 it asks for
             // about 45 cents of detune, a usual reese spread.
@@ -117,6 +140,7 @@ impl Default for Settings {
 }
 impl Settings {
     pub fn validate(&self) -> Result<(), &'static str> {
+        self.pitch.validate()?;
         let finite = [
             self.depth,
             self.low_cut,