test: replace jest workers past 200 MB so they exit in time (closes #766)

On a starved CPU, jest force-stopped worker processes that could not exit
within the 500 ms it allows, and script/lib/jest.js failed the run although
every test passed. No test leaks a handle; exit time grows with what a
worker holds, 380 to 970 MB at the end of the run.

workerIdleMemoryLimit in package.json replaces a worker whose heap is over
200 MB after a test file with a fresh one.

tests/jestRunner.test.js runs script/lib/jest.js on a fixture that leaves
an interval running and expects the force-stop to fail the run.

Model: opus-5-5

package.json

@@ -16,7 +16,8 @@
     "jest": {
         "setupFiles": [
             "<rootDir>/tests/support/navigatorLocks.js"
-        ]
+        ],
+        "workerIdleMemoryLimit": "200MB"
     },
     "devDependencies": {
         "@eslint/js": "10.0.1",