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

TODO.md

@@ -45,6 +45,17 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-04: `make test` runs jest in three worker processes
+  ([#426](https://git.eeqj.de/sneak/AutistMask/issues/426)). The `test` and
+  `test:verbose` scripts in `package.json` ran `jest --forceExit`, which starts
+  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`, and the suite takes 23-29s
+  there: inside the 30-second cap in `script/test`, which is unchanged, but not
+  by much, because `tests/persistedFieldContract.test.js` alone takes most of it
+  ([#428](https://git.eeqj.de/sneak/AutistMask/issues/428)). One or two workers
+  went past the cap. `make check`, the pre-commit hook and `script/cibuild` all
+  run the suite through these scripts.
+
 - 2026-10-04: The error container on each dApp approval screen keeps its height
   when an error appears
   ([#297](https://git.eeqj.de/sneak/AutistMask/issues/297)). `#approve-tx-error`