Adds persist_submit_job_array_async(): encodes one JobSubmit-shaped
payload (base jobid, array size, one shared spec), appends exactly
one WAL_JOB_ARRAY_SUBMIT entry, and registers exactly one commit
continuation regardless of array size -- reuses the existing
job_commit_adapter_ctx_t/job_commit_resolve_adapter machinery, no
per-element loop. Not yet wired into handle_jobsubmit (task 2.2).
Also fixes a test-helper bug (WAL offset back-computed from the
post-append cursor, only valid for zero-payload entries) that
corrupted state across tests when exercising a nonzero-payload
array-submit entry.
Task 2.1 of job-array-single-wal-record.
Rongsong Shen
7c4afbbb83