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/common/dsp/tests/blocks.rs

@@ -1,7 +1,7 @@
 use common_dsp::meter::HarmonicMeter;
 use common_dsp::oversample::Oversampler;
 use common_dsp::svf::{Svf, SvfCoef};
-use common_dsp::tracker::{NoteEvent, Tracker, TrackerConfig};
+use common_dsp::tracker::{NoteEvent, PitchMode, PitchSource, Scale, Tracker, TrackerConfig};
 use common_dsp::{hz_to_midi, Saw};
 use std::f32::consts::TAU;
 
@@ -174,7 +174,7 @@ fn harmonic_meter_reads_saw_harmonics() {
 #[test]
 fn tracker_reports_notes_at_output_time() {
     let mut t = Tracker::new(RATE, TrackerConfig::BASS);
-    t.set(false, -42., false);
+    t.set(PitchSource::default());
     let l = t.latency();
     assert!(l < (0.08 * RATE) as usize, "{l}");
     let onset = 4800;
@@ -210,7 +210,11 @@ fn tracker_reports_notes_at_output_time() {
 #[test]
 fn tracker_delays_midi_by_the_latency() {
     let mut t = Tracker::new(RATE, TrackerConfig::BASS);
-    assert!(t.set(true, -42., true));
+    assert!(t.set(PitchSource {
+        mode: PitchMode::Midi,
+        glide: true,
+        ..PitchSource::default()
+    }));
     let l = t.latency();
     let mut events = vec![];
     for i in 0..l + 2000 {
@@ -248,3 +252,127 @@ fn tracker_delays_midi_by_the_latency() {
         ]
     );
 }
+
+#[test]
+fn stable_tracker_holds_its_notes_on_the_stem_corpus() {
+    let stable = PitchSource::default();
+    let bends = PitchSource {
+        bends: true,
+        ..stable
+    };
+    let (mut extra_stable, mut extra_bends) = (0., 0.);
+    for stem in common_dsp::stems::all(RATE) {
+        let s = common_dsp::stems::score(
+            RATE,
+            TrackerConfig::BASS,
+            stable,
+            &stem.samples,
+            &stem.truth,
+        );
+        let b =
+            common_dsp::stems::score(RATE, TrackerConfig::BASS, bends, &stem.samples, &stem.truth);
+        assert!(s.extra_starts <= 0.25, "{}: {s:?}", stem.name);
+        assert!(s.wrong <= 0.05, "{}: {s:?}", stem.name);
+        assert!(s.wrong <= b.wrong + 0.005, "{}: {s:?} vs {b:?}", stem.name);
+        extra_stable += s.extra_starts;
+        extra_bends += b.extra_starts;
+    }
+    // Follow bends keeps the fast 0.1 behaviour, which flickers on formant basses.
+    assert!(
+        extra_bends > 10. * extra_stable.max(1.),
+        "{extra_stable} vs {extra_bends}"
+    );
+}
+
+#[test]
+fn tracker_turns_inactive_when_reads_stop_agreeing() {
+    let mut t = Tracker::new(RATE, TrackerConfig::BASS);
+    t.set(PitchSource::default());
+    let mut saw = Saw::new(RATE, 55.);
+    let mut seed = 1u32;
+    let mut active = vec![];
+    for i in 0..(RATE * 1.5) as usize {
+        seed ^= seed << 13;
+        seed ^= seed >> 17;
+        seed ^= seed << 5;
+        let noise = seed as f32 / u32::MAX as f32 - 0.5;
+        let x = if i < 24000 { 0.5 * saw.sample() } else { noise };
+        t.process(x, |_| {});
+        active.push(t.active());
+    }
+    let l = t.latency();
+    assert!(active[l + 12000..l + 23000].iter().all(|a| *a));
+    // Unreadable sound is no evidence against the note, so trust fades slowly: growls read
+    // only now and then. Within half a second of noise, tracked processing stops acting.
+    assert!(active[l + 48000..].iter().all(|a| !*a));
+}
+
+#[test]
+fn note_mode_plays_the_set_note_on_each_onset() {
+    let mut t = Tracker::new(RATE, TrackerConfig::BASS);
+    t.set(PitchSource {
+        mode: PitchMode::Note,
+        note: 29,
+        ..PitchSource::default()
+    });
+    let mut saw = Saw::new(RATE, 220.);
+    let mut events = vec![];
+    for i in 0..48000 {
+        let x = if (4800..20000).contains(&i) || i >= 30000 {
+            0.5 * saw.sample()
+        } else {
+            0.
+        };
+        t.process(x, |e| events.push(e));
+        if i == 15000 {
+            assert!(t.active());
+        }
+    }
+    let starts: Vec<_> = events
+        .iter()
+        .filter_map(|e| match e {
+            NoteEvent::Start { pitch, .. } => Some(*pitch),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(starts, vec![29., 29.]);
+    assert!(!events.iter().any(|e| matches!(e, NoteEvent::Move { .. })));
+}
+
+#[test]
+fn key_lock_snaps_reads_to_the_scale() {
+    // F minor: F G Ab Bb C Db Eb. F#1 (30) leans to G1 when read sharp.
+    assert_eq!(Scale::Minor.snap(29, 30.4), 31.);
+    assert_eq!(Scale::Minor.snap(29, 29.6), 29.);
+    assert_eq!(Scale::Major.snap(0, 61.), 60.);
+    assert_eq!(Scale::Root.snap(29, 40.6), 41.);
+    assert_eq!(Scale::Root.snap(29, 35.4), 41.);
+    assert_eq!(Scale::Free.snap(29, 30.37), 30.37);
+}
+
+#[test]
+fn key_lock_keeps_a_drifting_wobble_on_its_root() {
+    let stem = common_dsp::stems::all(RATE)
+        .into_iter()
+        .find(|s| s.name == "wobble")
+        .unwrap();
+    let free = PitchSource::default();
+    let root = PitchSource {
+        note: 31,
+        scale: Scale::Root,
+        ..free
+    };
+    let f = common_dsp::stems::score(RATE, TrackerConfig::BASS, free, &stem.samples, &stem.truth);
+    let r = common_dsp::stems::score(RATE, TrackerConfig::BASS, root, &stem.samples, &stem.truth);
+    assert!(r.unsure + r.wrong < f.unsure + f.wrong, "{f:?} vs {r:?}");
+    assert!(r.wrong <= 0.02, "{r:?}");
+}
+
+#[test]
+fn note_names_parse_back() {
+    for n in 0..=127u8 {
+        assert_eq!(common_dsp::parse_note(&common_dsp::note_name(n)), Some(n));
+    }
+    assert_eq!(common_dsp::parse_note(" f1 "), Some(29));
+    assert_eq!(common_dsp::parse_note("H2"), None);
+}