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

script/test

@@ -1,13 +1,16 @@
 #!/bin/sh
 # script/test: run the test suite.
 #
-# The timeout bounds a hung suite; it is not a performance budget. On a
-# developer host the suite finishes in about 8s and REPO_POLICIES' 30s cap is
-# the bound. Inside the image the same suite also pays a cold jest cache and
-# shares the runner with the rest of the build, which is not what that budget
-# describes, so the Dockerfile raises the bound through
-# AUTISTMASK_TEST_TIMEOUT. A cap a healthy suite can trip on a cold cache
-# produces a red that means nothing, and teaches "just run it again".
+# jest runs two worker processes (package.json), not one per CPU core: on a
+# many-core shared host one per core took gigabytes of RAM per run.
+#
+# The timeout bounds a hung suite; it is not a performance budget. On the busy
+# shared build host the suite takes about 28s with two workers and
+# REPO_POLICIES' 30s cap is the bound. Inside the image the same suite also
+# pays a cold jest cache and shares the runner with the rest of the build,
+# which is not what that budget describes, so the Dockerfile raises the bound
+# through AUTISTMASK_TEST_TIMEOUT. A cap a healthy suite can trip on a cold
+# cache produces a red that means nothing, and teaches "just run it again".
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"