chore: run jest in two 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=2. One process took the suite past the
30-second cap in script/test, so two it is; the cap is unchanged.
make check, the pre-commit hook and script/cibuild all reach jest
through these scripts. The suite timings quoted in the script/test and
Dockerfile comments are updated to match.

Model: opus-5-5

Dockerfile

@@ -9,7 +9,7 @@ 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
+# runs in about 28s 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
 # still catches a hang in three minutes and cannot be tripped by a suite that