ci: cap Gradle workers so test JVMs fit the container

.github/workflows/tests.yml

@@ -23,7 +23,13 @@ env:
   # make two builds likelier to coexist; the real fix is sizing the containers to the
   # host. See the "Report runner capacity" step.
   GRADLE_OPTS: -Dorg.gradle.jvmargs="-Xmx6g -XX:MaxMetaspaceSize=768m -Dfile.encoding=UTF-8 -XX:+HeapDumpOnOutOfMemoryError" -Dkotlin.daemon.jvm.options=-Xmx2g -XX:+UseG1GC
-  CI_GRADLE_ARG_PROPERTIES: --stacktrace --no-daemon -Dsonar.gradle.skipCompile=true --no-configuration-cache
+  # --max-workers caps how many test tasks run at once. build.gradle.kts sets
+  # maxParallelForks = cores/2 and screenshot tests fork at maxHeapSize=2g, so on these
+  # 8-core containers the default (workers = cores = 8) allows 8 x 4 x 2g = 64 GB of test
+  # JVMs against a 48 GiB cgroup limit, with no swap — the daemon gets OOM-killed and
+  # Gradle reports "the daemon has disappeared". GitHub's 4-core hosted runners reach only
+  # 2 x 2 x 2g = 16 GB and additionally have the 8 GB swapfile added above.
+  CI_GRADLE_ARG_PROPERTIES: --stacktrace --no-daemon -Dsonar.gradle.skipCompile=true --no-configuration-cache --max-workers=4
 
 jobs:
   tests: