feat(job-array-single-wal-record): harden jobj.c job construction primitives

- job_init() now checks makeJobsub()/makeString(ckptinfo) return
  values and propagates failure instead of storing NULL with a
  success return (Req 10.1).
- Root-caused the previously observed makeJobsub() NULL-with-EINVAL
  failure to a test-binary-only hazard: multiple controller test
  files each called job_obj_init() from their own setUp() with no
  cross-file coordination, re-running jobsub_table_init() against an
  already-live lock/table. Fixed via atomic_flag-guarded idempotent
  job_obj_init()/job_obj_fini(), plus a distinct -ENODEV return from
  makeJobsub()/makeJobstartinfo() when the table isn't ready
  (previously fell through to undefined behavior) (Req 10.2).
- Verified (test) MOVE_PTR fields are always NULL on a freshly-built
  submission payload -- no code change needed (Req 10.3).
- Fixed job_destroy()'s accounts leak via the existing
  free_job_account() helper, replacing the stale FIXME (Req 10.4).
- Verified (inspection + test) struct Job's capnp schema has no
  ckptinfo field, so decode_Job/free_Job have nothing to leak there
  -- no code change needed (Req 10.5).

Independently re-verified: reviewer reproduced a pre-existing,
unrelated rtti-library leak-on-init-failure against a stashed
pre-task copy of jobj.c to confirm it predates this change.

Task 1.3 of job-array-single-wal-record.

src/lib/meson.build

@@ -163,6 +163,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_bitset.c',
                    'testsuite/test_kv.c',
                    'testsuite/test_jobj.c',
+                   'testsuite/test_jobj_hardening.c',
                    'testsuite/test_wobj.c',
                    'testsuite/test_qobj.c',
                    'testsuite/test_request.c',