Every early-exit branch in the worker_list scan either re-tested the same
node forever (bare `continue`) or returned out of the whole function on the
first DONE/EXITED worker, so job array elements past the head of the list
never got their on_job_start report. Capture `next` before touching each
node and always advance to it. Affects xcecd_virtual and real xcecd alike
(same DBUS_MANAGER-gated function).
Also records a roadmap follow-up (xcecd_virtual concurrent-job support,
MXJ > 1) discovered while investigating this bug.
