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/ideas.md

@@ -212,11 +212,16 @@ one period of the note, so heavy settings do not ripple at f0. See [Planish](pla
 
 ## Next for released plugins
 
-- **Anvil**: *Kick lock*: the kick through the sidechain ducks the sub on each
-  hit and restarts its phase so kick and sub sum (problem 6). With it, an
-  *offset audition*: a sweep of sub offsets against the kick (−30 to +30 ms,
-  then finer) and a marker for where the hit is heard, kept apart from the
-  low-band cancellation reading (problem 21). A sub shape from sine to rounded
+- **Anvil**: *Kick lock* and the *offset audition* are done (unreleased, see
+  [Anvil](anvil/README.md)): the kick through the sidechain ducks the sub and
+  restarts its phase at an offset (problem 6); the audition steps −30 to +30 ms,
+  4 kicks each, Keep takes the offset heard, and the low-band sum per offset is
+  shown apart from that choice (problem 21). Measured on a 55 Hz kick burst
+  against a 55 Hz sub: +2.4 dB at 0 ms, −4.6 dB at −10 ms, −5.9 dB at +10 ms,
+  and +1.8 to +2.2 dB again one period away (±20 ms), so the sum alone cannot
+  pick the hit. Open: a finer second sweep around the kept offset; a gate or
+  sensitivity for kicks with long tails or a kick bus with other drums.
+  A sub shape from sine to rounded
   triangle. A note attack counted in cycles. *Ghost harmonics* and the
   *Phone* listen are done (unreleased, see [Anvil](anvil/README.md)): harmonics
   2–10 above a "from" frequency, filled only where the patch lacks them, with
@@ -264,7 +269,7 @@ one period of the note, so heavy settings do not ripple at f0. See [Planish](pla
      spread (done, unreleased); the fatness lane, as the first Assay piece
      (prototype `scripts/fatness.py`); Anvil Ghost harmonics and Phone listen
      (done, unreleased); Braze (done, unreleased); Anvil Kick lock with offset
-     audition.
+     audition (done, unreleased).
 4. Forge, when enough modules exist.
 5. Assay, one plugin for the display-only ideas.
 6. Texture plugins (Anneal, Swarf, Etch, Bellows) and the note-keyed sequencer.