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/ballast/core/src/lib.rs

@@ -11,7 +11,7 @@
 mod settings;
 use common_dsp::loudness::{lufs, KWeighting};
 use common_dsp::svf::{Svf, SvfCoef};
-use common_dsp::tracker::{NoteEvent, Tracker, TrackerConfig};
+use common_dsp::tracker::{NoteEvent, PitchMode, PitchSource, Tracker, TrackerConfig};
 use common_dsp::{coefficient, db_to_gain, gain_to_db};
 pub use settings::{
     Grid, Segment, Settings, Source, FROM_MAX, FROM_MIN, RANGE_MAX, TARGET_MAX, TARGET_MIN,
@@ -216,7 +216,15 @@ impl Engine {
     }
     pub fn set_settings(&mut self, s: Settings) -> Result<(), &'static str> {
         s.validate()?;
-        self.tracker.set(s.source == Source::Midi, s.gate, false);
+        self.tracker.set(PitchSource {
+            mode: if s.source == Source::Midi {
+                PitchMode::Midi
+            } else {
+                PitchMode::Audio
+            },
+            gate_db: s.gate,
+            ..PitchSource::default()
+        });
         if s.from != self.settings.from || self.fresh {
             self.from_coef = if s.from > FROM_MIN {
                 SvfCoef::highpass(self.rate, s.from, Q)

plugins/ballast/core/src/settings.rs

@@ -22,9 +22,10 @@ pub enum Source {
 #[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq, Default)]
 pub enum Segment {
     /// Every note start.
-    #[default]
     Notes,
     /// Every line of the host's beat grid, so a patch change on the same note counts too.
+    /// The default: a switch-up lands on the grid, and the grid needs no note tracking.
+    #[default]
     Grid,
 }
 
@@ -91,7 +92,7 @@ impl Default for Settings {
     fn default() -> Self {
         Self {
             source: Source::Audio,
-            segment: Segment::Notes,
+            segment: Segment::Grid,
             grid: Grid::Quarter,
             target: -14.,
             amount: 100.,

plugins/ballast/core/tests/dsp.rs

@@ -117,6 +117,7 @@ fn note_starts_from_the_audio_cut_the_parts() {
     let mut e = Engine::new(RATE, 2).unwrap();
     e.set_settings(Settings {
         target,
+        segment: Segment::Notes,
         ..Settings::default()
     })
     .unwrap();
@@ -151,6 +152,7 @@ fn midi_note_starts_cut_legato_parts() {
     e.set_settings(Settings {
         target,
         source: Source::Midi,
+        segment: Segment::Notes,
         ..Settings::default()
     })
     .unwrap();