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.

docs/chain.md

@@ -29,7 +29,8 @@ bass bus:     Anvil → (bus compression, clipping) → Crucible
   layer but cannot move it earlier. If FL Studio does not compensate the
   sidechain route, any latency added after Weld's input on the main layer makes
   that layer late.
-- **Anvil on the bus, in MIDI mode when Knurl is in the chain.**
+- **Anvil on the bus, in MIDI mode when Knurl is in the chain.** For Kick
+  lock, send the kick's track to Anvil as a sidechain.
 - **Crucible at the end of the chain you want to resample.**
 
 ## Knurl and pitch detection