Adds Data union variant jobArray@6, reusing JobSubmit's existing
(jobid, arraySize, submit) shape verbatim -- no new struct, no
existing variant renumbered. Not yet wired into any push path
(tasks 5.1/5.2 own that).
Task 1.2 of job-array-single-wal-record.
Adds the array-shaped WAL entry type (distinct from WAL_JOB_SUBMIT,
value 11 in the unused WAL_JOB_* gap) and classifies it WAIT-mode via
wal_classify, matching the single-job type's commit-barrier semantics.
No dispatch/apply wiring yet -- foundation for task 2.x/3.x.
Task 1.1 of job-array-single-wal-record.
