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/lathe/core/tests/dsp.rs

@@ -137,3 +137,44 @@ fn settings_are_validated_and_serialise() {
     assert!(e.set_settings(bad).is_err());
     assert_eq!(e.settings(), s);
 }
+
+#[test]
+fn note_mode_cuts_the_set_notes_harmonics() {
+    let mut s = Settings::default();
+    s.gains[2] = -12.;
+    s.source = Source::Note;
+    s.pitch.note = 29;
+    let hz = 43.65;
+    let x = saw(hz, 1., 0.5);
+    let (mut e, l) = engine(s);
+    let y = render(&mut e, &x);
+    let a = harmonics(&x[..x.len() - l], hz);
+    let b = harmonics(&y[l..], hz);
+    assert!((b[2] - a[2] + 12.).abs() < 0.7, "{a:?} {b:?}");
+    assert!((b[1] - a[1]).abs() < 1.);
+}
+
+#[test]
+fn unsure_tracking_passes_the_audio_through() {
+    let mut s = Settings::default();
+    s.gains[0] = -12.;
+    s.gains[1] = -12.;
+    s.gains[2] = -12.;
+    let (mut e, l) = engine(s);
+    // A clear note, then noise detection cannot read.
+    let mut x = saw(55., 0.5, 0.5);
+    let mut seed = 7u32;
+    for _ in 0..RATE as usize {
+        seed ^= seed << 13;
+        seed ^= seed >> 17;
+        seed ^= seed << 5;
+        x.push((seed as f32 / u32::MAX as f32 - 0.5) * 0.5);
+    }
+    let y = render(&mut e, &x);
+    let tail = (RATE * 1.0) as usize;
+    let mut worst = 0f32;
+    for i in tail..x.len() {
+        worst = worst.max((y[i] - x[i - l]).abs());
+    }
+    assert!(worst < 1e-3, "{worst}");
+}