feat(rivet): storyboard composition with flexible lengths and meters

Rivet 0.4.0 tells a motif as a story. A timeline of blocks with roles
(statement, answer, departure, return, rest) replaces tasks, pins and
branches. Rivet writes each block from a motif and joins the seams with
a cut, breath, pickup or approach.

- Motifs of 1-8 bars in a bin of 16; storyboards up to 32 bars.
- Meters 2/4, 3/4, 4/4, 5/4, 6/8, 7/8 and 12/8 for import, writing,
  audition and export. Phrygian, harmonic minor and Mixolydian scales.
- Per-block options, Another take and Keep; templates for new boards.
- Editor rebuilt: motif bin, timeline, large piano roll, inspector,
  tabs for generation, harmony and sound roles, 32-step undo/redo.
- Sessions from 0.1-0.3 open as a storyboard from their source motif.

plugins/rivet/core/src/performance.rs

@@ -1,5 +1,5 @@
 //! Explicit sound roles and controller gestures, compiled away from the audio thread.
-use crate::{err, Note, Phrase, Result, BAR, MAX_PLAYBACK_NOTES, PPQ, SECTION};
+use crate::{err, Meter, Note, Phrase, Result, MAX_LENGTH, MAX_PLAYBACK_NOTES, PPQ};
 use serde::{Deserialize, Serialize};
 use std::f64::consts::TAU;
 
@@ -28,12 +28,14 @@ impl RoleMode {
             Self::Interlock => "Strong beats and replies",
         }
     }
-    pub fn role(self, index: usize, n: &Note) -> usize {
+    /// Which of the two voices plays note `index`. Question/answer alternates bars;
+    /// strong beats follow the meter's pulse.
+    pub fn role(self, index: usize, n: &Note, meter: Meter) -> usize {
         match self {
             Self::Solo => 0,
             Self::Alternating => index % 2,
-            Self::CallResponse => usize::from(n.start % SECTION >= BAR),
-            Self::Interlock => usize::from(!n.start.is_multiple_of(PPQ)),
+            Self::CallResponse => ((n.start / meter.bar()) % 2) as usize,
+            Self::Interlock => usize::from(!meter.strong(n.start)),
         }
     }
 }
@@ -224,6 +226,7 @@ impl Performance {
         settings: &PerformanceSettings,
         bpm: f64,
         compensate: bool,
+        meter: Meter,
     ) -> Result<Self> {
         phrase.validate(MAX_PLAYBACK_NOTES)?;
         settings.validate()?;
@@ -252,7 +255,7 @@ impl Performance {
                 .notes
                 .iter()
                 .enumerate()
-                .filter(|(i, n)| settings.mode.role(*i, n) == role)
+                .filter(|(i, n)| settings.mode.role(*i, n, meter) == role)
                 .map(|(_, n)| n)
                 .collect();
             if routed.is_empty() && !profile.controller.enabled {
@@ -378,7 +381,7 @@ impl Performance {
     pub fn clip(&self) -> Result<PerformanceClip> {
         if self.events.len() > MAX_PERFORMANCE_EVENTS
             || self.length == 0
-            || self.length > 8 * BAR
+            || self.length > MAX_LENGTH
             || self.lead_in >= self.length
         {
             return Err(err("Invalid performance clip."));