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/ui/Cargo.toml

@@ -6,3 +6,4 @@ license.workspace = true
 
 [dependencies]
 egui = "0.31.1"
+common-dsp = { path = "../dsp" }

plugins/common/ui/src/lib.rs

@@ -1,4 +1,6 @@
 //! Shared hardware look for every plugin editor: palette, fonts, knobs, panels.
+use common_dsp::note_name;
+use common_dsp::tracker::{Pitch, PitchMode, Scale, NOTE_MAX, NOTE_MIN};
 use egui::{self, Color32, FontId, Pos2, Rect, RichText, Sense, Stroke, Vec2};
 
 // Dark anodized hardware: graphite surfaces, one signal colour, steel data traces.
@@ -359,3 +361,168 @@ pub fn meter(ui: &mut egui::Ui, label: &str, db: f32, color: Color32, width: f32
         );
     });
 }
+
+const PITCH_CLASSES: [&str; 12] = [
+    "C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B",
+];
+fn scale_name(scale: Scale) -> &'static str {
+    match scale {
+        Scale::Free => "off",
+        Scale::Minor => "minor",
+        Scale::Major => "major",
+        Scale::Root => "root only",
+    }
+}
+/// Short status of the pitch source for readout lines: "detecting pitch in F minor".
+pub fn pitch_status(mode: PitchMode, pitch: &Pitch) -> String {
+    match (mode, pitch.scale) {
+        (PitchMode::Audio, Scale::Free) => "detecting pitch".into(),
+        (PitchMode::Audio, s) => format!(
+            "detecting pitch, key {} {}",
+            PITCH_CLASSES[pitch.note as usize % 12],
+            scale_name(s)
+        ),
+        (PitchMode::Midi, _) => "following MIDI".into(),
+        (PitchMode::Note, _) => format!("fixed on {}", note_name(pitch.note)),
+    }
+}
+/// Pitch source switch for a header's right-to-left row: AUDIO, MIDI and NOTE, then a button
+/// that opens Note mode's note, or the key lock and Follow bends for detection.
+pub fn pitch_from(ui: &mut egui::Ui, mode: &mut PitchMode, pitch: &mut Pitch) {
+    let id = ui.make_persistent_id("pitch-detail");
+    let detail = match *mode {
+        PitchMode::Note => Some(note_name(pitch.note)),
+        PitchMode::Audio => Some(match pitch.scale {
+            Scale::Free if pitch.bends => "BENDS".into(),
+            Scale::Free => "KEY OFF".into(),
+            s => format!(
+                "{} {}",
+                PITCH_CLASSES[pitch.note as usize % 12],
+                match s {
+                    Scale::Minor => "MIN",
+                    Scale::Major => "MAJ",
+                    _ => "ROOT",
+                }
+            ),
+        }),
+        PitchMode::Midi => None,
+    };
+    if let Some(detail) = detail {
+        let help = if *mode == PitchMode::Note {
+            "The note this plugin follows. Nothing is detected: set the drop's root."
+        } else {
+            "Key lock and Follow bends for detection."
+        };
+        let r = ui
+            .button(RichText::new(detail).size(13.).color(ACCENT))
+            .on_hover_text(help);
+        if r.clicked() {
+            ui.memory_mut(|m| m.toggle_popup(id));
+        }
+        let note_mode = *mode == PitchMode::Note;
+        egui::popup_below_widget(
+            ui,
+            id,
+            &r,
+            egui::PopupCloseBehavior::CloseOnClickOutside,
+            |ui| {
+                ui.set_min_width(220.);
+                if note_mode {
+                    pitch_note(ui, pitch, true);
+                } else {
+                    caption(ui, "KEY LOCK");
+                    ui.horizontal(|ui| {
+                        for s in [Scale::Free, Scale::Minor, Scale::Major, Scale::Root] {
+                            let help = match s {
+                                Scale::Free => "Any pitch, also between notes.",
+                                Scale::Minor => "Only notes of the natural minor scale.",
+                                Scale::Major => "Only notes of the major scale.",
+                                Scale::Root => "Only the root: detection picks the octave.",
+                            };
+                            let label = RichText::new(scale_name(s).to_uppercase()).size(12.);
+                            if ui
+                                .selectable_label(pitch.scale == s, label)
+                                .on_hover_text(help)
+                                .clicked()
+                            {
+                                pitch.scale = s;
+                            }
+                        }
+                    });
+                    if pitch.scale != Scale::Free {
+                        ui.add_space(4.);
+                        caption(ui, "ROOT");
+                        pitch_note(ui, pitch, false);
+                    }
+                    ui.add_space(6.);
+                    ui.checkbox(&mut pitch.bends, "Follow bends").on_hover_text(
+                        "Retune on every bend and switch notes within 10 ms. Fast, but it \
+                         flickers on growls. Off holds each note until a new one is clear.",
+                    );
+                }
+            },
+        );
+    }
+    for (m, label, help) in [
+        (
+            PitchMode::Note,
+            "NOTE",
+            "You set the note; the audio only starts and stops it. Nothing is detected.",
+        ),
+        (
+            PitchMode::Midi,
+            "MIDI",
+            "Notes from MIDI input. In FL, set the channel's MIDI output port and this \
+             effect's input port to the same number.",
+        ),
+        (
+            PitchMode::Audio,
+            "AUDIO",
+            "Detect the pitch from the audio. When detection is unsure, tracked processing \
+             steps back instead of guessing.",
+        ),
+    ] {
+        let r = ui
+            .selectable_label(*mode == m, RichText::new(label).size(13.))
+            .on_hover_text(help);
+        if r.clicked() {
+            *mode = m;
+        }
+    }
+    caption(ui, "PITCH FROM");
+}
+/// Twelve pitch-class buttons, and octave steps when `octave` is set.
+fn pitch_note(ui: &mut egui::Ui, pitch: &mut Pitch, octave: bool) {
+    let class = pitch.note % 12;
+    let base = pitch.note - class;
+    for row in [0..6, 6..12] {
+        ui.horizontal(|ui| {
+            for c in row {
+                let r = ui.add_sized(
+                    [30., 22.],
+                    egui::SelectableLabel::new(class == c as u8, PITCH_CLASSES[c]),
+                );
+                if r.clicked() {
+                    pitch.note = (base + c as u8).clamp(NOTE_MIN, NOTE_MAX);
+                }
+            }
+        });
+    }
+    if octave {
+        ui.horizontal(|ui| {
+            caption(ui, "OCTAVE");
+            if ui.button("−").clicked() && pitch.note >= NOTE_MIN + 12 {
+                pitch.note -= 12;
+            }
+            ui.label(
+                RichText::new(format!("{}", pitch.note as i32 / 12 - 1))
+                    .monospace()
+                    .color(TEXT),
+            );
+            if ui.button("+").clicked() && pitch.note + 12 <= NOTE_MAX {
+                pitch.note += 12;
+            }
+            ui.label(RichText::new(note_name(pitch.note)).color(ACCENT));
+        });
+    }
+}