chore: run jest in three worker processes (closes #426)

The test and test:verbose scripts ran jest with one worker per CPU core,
about 47 processes and 7-8 GiB per run on the shared 48-core build host.
They now pass --maxWorkers=3. On that host the suite takes 23-29s, inside
the unchanged 30-second cap in script/test but not by much: one test
file, tests/persistedFieldContract.test.js, takes most of it. One or two
workers went past the cap, so this departs from the issue's two-process
limit. make check, the pre-commit hook and script/cibuild all reach jest
through these scripts; the timings in the script/test and Dockerfile
comments are updated to match.

Model: opus-5-5

Dockerfile

@@ -9,9 +9,9 @@ WORKDIR /app
 ENV AUTISTMASK_LINT_NATIVE=1
 
 # script/test's default 30s bound is the host figure, against a suite that
-# runs in about 8s there. In here the same suite starts on a cold jest cache
-# and shares the runner with the rest of the build, so 30s is marginal rather
-# than a bound — it killed a healthy suite at 30.6s on a cold CI cache. 180s
+# takes 23-29s there with three jest workers. In here the same suite starts on
+# a cold jest cache and shares the runner with the rest of the build, so 30s
+# is too tight — it killed a healthy suite at 30.6s on a cold CI cache. 180s
 # still catches a hang in three minutes and cannot be tripped by a suite that
 # is merely running on contended hardware.
 ENV AUTISTMASK_TEST_TIMEOUT=180