feat(anvil): add Kick lock with offset audition

Anvil gets a sidechain input named Kick. With Kick lock on, kicks are
found on the undelayed sidechain, so with the lookahead each hit ducks
the sub (Duck, Release) when the delayed kick plays and restarts the sub
at Phase, shifted by Offset (-30 to +30 ms, also before the kick).

The Audition steps the offset from -30 to +30 ms, 4 kicks each, and
measures how kick and sub sum below 150 Hz after each hit; Keep takes the
offset heard. On a 55 Hz kick burst against a 55 Hz sub the sum peaks at
0 ms (+2.4 dB) and nearly again one period away (+/-20 ms), so the bars
check the choice by ear rather than make it.

plugins/anvil/core/src/settings.rs

@@ -44,8 +44,30 @@ pub struct Settings {
     /// Round detected pitch to the nearest semitone.
     pub snap: bool,
     pub bypass: bool,
+    /// Kick lock: on each kick in the sidechain, duck the sub and restart its phase.
+    #[serde(default)]
+    pub kick_lock: bool,
+    /// How far the sub dips on each kick, in dB.
+    #[serde(default = "default_kick_duck")]
+    pub kick_duck: f32,
+    /// Time for the sub to come back after a kick, in ms.
+    #[serde(default = "default_kick_release")]
+    pub kick_release: f32,
+    /// When the sub restarts against the kick's start, in ms. Negative: before the kick.
+    #[serde(default)]
+    pub kick_offset: f32,
 }
 const GHOST_FROM: f32 = 300.;
+const KICK_DUCK: f32 = 6.;
+const KICK_RELEASE: f32 = 80.;
+/// Range of the kick offset, in ms either side of the kick.
+pub const KICK_OFFSET_MAX: f32 = 30.;
+fn default_kick_duck() -> f32 {
+    KICK_DUCK
+}
+fn default_kick_release() -> f32 {
+    KICK_RELEASE
+}
 fn default_ghost_from() -> f32 {
     GHOST_FROM
 }
@@ -68,6 +90,10 @@ impl Default for Settings {
             retrigger: true,
             snap: true,
             bypass: false,
+            kick_lock: false,
+            kick_duck: KICK_DUCK,
+            kick_release: KICK_RELEASE,
+            kick_offset: 0.,
         }
     }
 }
@@ -85,6 +111,9 @@ impl Settings {
             self.gate,
             self.keep,
             self.output,
+            self.kick_duck,
+            self.kick_release,
+            self.kick_offset,
         ]
         .iter()
         .all(|v| v.is_finite());
@@ -102,7 +131,10 @@ impl Settings {
             && (0.0..=360.).contains(&self.phase)
             && (-60.0..=-20.).contains(&self.gate)
             && (0.0..=100.).contains(&self.keep)
-            && (-12.0..=12.).contains(&self.output);
+            && (-12.0..=12.).contains(&self.output)
+            && (0.0..=24.).contains(&self.kick_duck)
+            && (10.0..=300.).contains(&self.kick_release)
+            && (-KICK_OFFSET_MAX..=KICK_OFFSET_MAX).contains(&self.kick_offset);
         if ok {
             Ok(())
         } else {