test(runtime): assert shutdown_impl return value in write failure test

[rocketman-code]

The shutdown_impl_reports_write_errors_to_stderr unit test calls shutdown_impl but discards the return value. Since shutdown_impl now returns bool to signal write failure, the test should assert:

let failed = shutdown_impl(&tmp);
assert!(failed, "shutdown_impl should return true when writes fail");

This would verify the documented contract rather than just checking for no panic. One-line fix.