refactor: run Lathe, Temper and Knurl as tracked modules

Add a Tracked trait and a Lookahead harness to common-dsp. Each engine is
now its tracker plus a module that follows note events aligned with its
input, so the same module can run behind a shared tracker.

Lathe and Temper output is bit-exact against the old engines. Knurl's
interpolation now reads one newer sample inside the module, so its
latency is one sample longer; the output matches within float rounding.

Each editor exposes body() and toggles() besides show(), so another
editor can draw a module's page.

plugins/common/dsp/src/tracker.rs

@@ -397,3 +397,55 @@ impl PitchFollower {
         self.hz
     }
 }
+
+/// A processor driven by note events that are aligned with its input audio. Lathe, Temper and
+/// Knurl are built from one each, so the same code runs standalone (after its own tracker) and
+/// inside a strip that shares one tracker between several of them.
+pub trait Tracked {
+    /// Delay of the output behind the input, in samples.
+    fn latency(&self) -> usize;
+    /// The undelayed input, one tracker lookahead ahead of the audio passed to `process`.
+    /// Called before the sample's events.
+    fn ahead(&mut self, _mono: f32) {}
+    /// A note event due at the next `process` sample.
+    fn event(&mut self, e: NoteEvent);
+    /// One sample of audio; `sounding` is the tracker's note state at this sample.
+    fn process(&mut self, x: [f32; 2], sounding: bool) -> [f32; 2];
+}
+
+/// A tracker and the lookahead delay in front of one [`Tracked`] module: the standalone form.
+pub struct Lookahead {
+    pub tracker: Tracker,
+    delay: Vec<[f32; 2]>,
+    at: usize,
+}
+impl Lookahead {
+    pub fn new(rate: f32, cfg: TrackerConfig) -> Self {
+        let tracker = Tracker::new(rate, cfg);
+        let len = tracker.latency() + 1;
+        Self {
+            tracker,
+            delay: vec![[0.; 2]; len],
+            at: 0,
+        }
+    }
+    /// The tracker's lookahead, in samples.
+    pub fn latency(&self) -> usize {
+        self.tracker.latency()
+    }
+    pub fn reset(&mut self) {
+        self.tracker.reset();
+        self.delay.fill([0.; 2]);
+    }
+    /// Feeds one input sample (`mono` is its channel average) and returns `module`'s output.
+    #[inline]
+    pub fn process(&mut self, x: [f32; 2], mono: f32, module: &mut impl Tracked) -> [f32; 2] {
+        module.ahead(mono);
+        self.tracker.process(mono, |e| module.event(e));
+        let len = self.delay.len();
+        self.delay[self.at] = x;
+        self.at = (self.at + 1) % len;
+        let y = self.delay[self.at];
+        module.process(y, self.tracker.sounding())
+    }
+}