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

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 three 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 23-29s with three workers, so
+# REPO_POLICIES' 30s cap is tight there, not comfortable. 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)"