Unknown reserved __omp_worker_* selectors hit the worker-host re-entry
seam, runWorkerEntrypoint returned false, and the ignored result let the
process exit 0 with empty stdout/stderr. A stale or mistyped selector
looked healthy to parent processes and install smoke paths.
Check runWorkerEntrypoint's return at the dispatch seam: an unrecognized
selector now writes "Error: unknown worker selector: <arg>" to stderr and
sets a nonzero exit code without starting a worker. Known selectors and
normal CLI arguments are unchanged.
Fixes#5712
roboomp