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/plugin/tests/host.rs

@@ -97,28 +97,22 @@ fn exported_factory_state_and_silent_audio_lifecycle() {
         };
         let mut envelope: nih_plug::prelude::PluginState =
             serde_json::from_slice(&save()[8..]).unwrap();
-        let mut session = rivet_core::Session {
-            source: Some(rivet_core::demo_motif()),
-            tempo: 150.,
-            ..Default::default()
-        };
-        session.source.as_mut().unwrap().notes[1].locked = true;
-        session.harmony = rivet_core::Harmony::progression(0, rivet_core::Scale::Minor);
+        let mut session = rivet_core::Session::demo();
+        session.tempo = 150.;
+        session.motifs[0].phrase.notes[1].locked = true;
+        session.settings.meter = rivet_core::Meter { beats: 6, unit: 8 };
+        session.harmony =
+            rivet_core::Harmony::progression(0, rivet_core::Scale::Minor, session.settings.meter);
         session.audition = rivet_core::Audition::Midi;
-        session.regenerate().unwrap();
-        session.pin_selected(1).unwrap();
-        session.accept();
-        session.selected = 2;
-        session.accept();
+        session.realize_all();
+        session.blocks[1].choice = 1;
+        session.blocks[1].kept = true;
+        session.blocks[2].join = rivet_core::Join::Breath;
+        session.another_take(3);
         session.performance.mode = rivet_core::RoleMode::CallResponse;
         session.performance.voices[1].controller.enabled = true;
         session.performance.voices[1].controller.cc = 20;
         session.performance.voices[1].attack_ms = 75;
-        session.save_gesture("Airy question", None).unwrap();
-        session.save_branch("Original call").unwrap();
-        session
-            .apply_move(rivet_core::CreativeMove::Thin, Some(0))
-            .unwrap();
         envelope.fields.insert(
             "story-session-v1".into(),
             String::from_utf8(session.to_json().unwrap()).unwrap(),