feat: add tempo-sync for delay time

Add a Sync checkbox parameter.  When enabled the Time parameter is
interpreted as a beat subdivision (1.0 = quarter note, 0.5 = eighth,
2.0 = half note) derived from the host transport tempo.  Falls back to
120 BPM when no tempo is available.  The Time label shows 'Time (1/n)'
in the UI when sync is on for visual clarity.

src/lib.rs

@@ -5,7 +5,7 @@ use nih_plug_egui::{create_egui_editor, egui, widgets, EguiState};
 use std::sync::Arc;
 
 // ──────────────────────────────────────────────
-// Params — time, feedback, mix
+// Params — time, feedback, mix, sync
 // ──────────────────────────────────────────────
 
 const MAX_DELAY_MS: f32 = 2_000.0;
@@ -16,7 +16,7 @@ struct KolmekiviParams {
     #[persist = "editor-state"]
     editor_state: Arc<EguiState>,
 
-    /// Delay time in milliseconds.
+    /// Delay time in milliseconds (sync off) or beat subdivision (sync on).
     #[id = "time"]
     time: FloatParam,
 
@@ -27,12 +27,17 @@ struct KolmekiviParams {
     /// Wet/dry mix (0 = dry only, 1 = wet only).
     #[id = "mix"]
     mix: FloatParam,
+
+    /// When true, Time is interpreted as a beat subdivision (1.0 = quarter note)
+    /// derived from the host tempo.  When false, Time is raw milliseconds.
+    #[id = "sync"]
+    sync: BoolParam,
 }
 
 impl Default for KolmekiviParams {
     fn default() -> Self {
         Self {
-            editor_state: EguiState::from_size(320, 160),
+            editor_state: EguiState::from_size(320, 180),
             time: FloatParam::new(
                 "Time",
                 500.0,
@@ -54,6 +59,8 @@ impl Default for KolmekiviParams {
                 FloatRange::Linear { min: 0.0, max: 1.0 },
             )
             .with_smoother(SmoothingStyle::Logarithmic(32.0)),
+
+            sync: BoolParam::new("Sync", false),
         }
     }
 }
@@ -168,9 +175,14 @@ impl Plugin for Kolmekivi {
             (),
             |_, _| {},
             move |egui_ctx, setter, _state| {
+                let sync = params.sync.value();
                 egui::CentralPanel::default().show(egui_ctx, |ui| {
                     ui.horizontal(|ui| {
-                        ui.label("Time");
+                        if sync {
+                            ui.label("Time (1/n)");
+                        } else {
+                            ui.label("Time");
+                        }
                         ui.add(widgets::ParamSlider::for_param(&params.time, setter));
                     });
                     ui.horizontal(|ui| {
@@ -181,6 +193,14 @@ impl Plugin for Kolmekivi {
                         ui.label("Mix");
                         ui.add(widgets::ParamSlider::for_param(&params.mix, setter));
                     });
+                    ui.horizontal(|ui| {
+                        let mut sync_val = params.sync.value();
+                        let changed = ui.checkbox(&mut sync_val as &mut bool, "Sync to tempo");
+                        if changed.changed() {
+                            setter.begin_set_parameter(&params.sync);
+                            setter.set_parameter(&params.sync, sync_val);
+                        }
+                    });
                 });
             },
         )
@@ -222,7 +242,20 @@ impl Plugin for Kolmekivi {
         let time_val = self.params.time.smoothed.next();
         let feedback = self.params.feedback.smoothed.next();
         let mix = self.params.mix.smoothed.next();
-        let delay_samples = (time_val * sample_rate / 1000.0) as usize;
+        let sync = self.params.sync.value();
+
+        let delay_samples = if sync {
+            // Time is a beat subdivision multiplier: 1.0 = quarter note,
+            // 0.5 = eighth, 2.0 = half note, etc.
+            let tempo = _context.transport().tempo.unwrap_or(120.0) as f32;
+            let beat_duration_s = 60.0_f32 / tempo; // quarter note in seconds
+            let subdivisions = time_val.clamp(0.015625, 8.0); // 32nd to 4x whole
+            let delay_s = subdivisions * beat_duration_s;
+            (delay_s * sample_rate) as usize
+        } else {
+            // Time is raw milliseconds
+            (time_val * sample_rate / 1000.0) as usize
+        };
 
         // Process each sample in the buffer
         for mut frame in buffer.iter_samples() {