Summary

• add ZIOAppMainSpec in core-tests/jvm-native to validate shutdown-flow behavior through main
• cover simulated external-shutdown path to ensure interruption produces failure exit and app finalizers run
• cover gracefulShutdownTimeout behavior by verifying shutdown hook returns quickly even when finalizers are still running
• add a small test seam in ZIOAppPlatformSpecific (registerShutdownHook) so shutdown hooks can be injected/observed in tests without changing runtime behavior

Why this lane

This targets the lifecycle areas called out in #9909 and the linked historical regressions around shutdown/finalizer behavior (#9901, #9807).

Test commands

sbt 'coreTestsJVM/testOnly zio.ZIOAppMainSpec'
sbt 'coreTestsJVM/testOnly zio.ZIOAppSpec'

Notes

• tests are JVM-specific and live under core-tests/jvm-native
• the hook-timeout spec intentionally verifies bounded wait behavior while finalization is still in progress

/claim #9909