Split mark_job_start/finish/fail/update in job.c into four hook-free
core functions (apply_job_*_transition) containing only the
monotonic-rank guard and mutation. Wrappers now peek the same guard
to decide hook-firing, then delegate to the core, preserving exact
pre-extraction hook-firing timing. Prerequisite for task 2.2's
apply_committed_entry() dispatch and the ordered apply pump (6.1).
