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/temper/core/src/lib.rs

@@ -7,7 +7,7 @@ mod settings;
 use common_dsp::meter::{HarmonicMeter, HARMONICS};
 use common_dsp::oversample::Oversampler;
 use common_dsp::svf::{Svf, SvfCoef};
-use common_dsp::tracker::{Lookahead, NoteEvent, PitchFollower, Tracked, TrackerConfig};
+use common_dsp::tracker::{Lookahead, NoteEvent, NoteState, PitchFollower, Tracked, TrackerConfig};
 use common_dsp::{coefficient, db_to_gain, gain_to_db, hz_to_midi};
 pub use settings::{Curve, Settings, Source};
 
@@ -381,8 +381,10 @@ impl Tracked for Module {
         self.pitch.event(false, e);
         self.level.event(e);
     }
-    fn process(&mut self, y: [f32; 2], sounding: bool) -> [f32; 2] {
-        self.sounding = sounding;
+    fn process(&mut self, y: [f32; 2], note: NoteState) -> [f32; 2] {
+        // Unsure tracking sends no retune, so Floor, Push and Bright hold their last sure
+        // tuning; only the readout drops the note.
+        self.sounding = note.sure;
         self.run(y)
     }
 }
@@ -424,7 +426,7 @@ impl Engine {
     fn set_tracker(&mut self, s: &Settings) {
         self.lookahead
             .tracker
-            .set(s.source == Source::Midi, s.gate, true);
+            .set(s.pitch.source(s.source.mode(), s.gate, true));
     }
     pub fn reset(&mut self) {
         self.lookahead.reset();