fix(controller): WAL_JOB_CKPT loses job.ckptinfo on replay (bug-1691)

Same class as bug-1678/1687/1689/1690: on_job_ckpt() sets job.ckptinfo
(a plain char*, xstring-backed via makeString()) but is purely hook-driven
-- persist_job_hook_fun's default: arm only ever captured {jobid, state}
for WAL_JOB_CKPT. Neither apply seam restored ckptinfo after a restart.

Simplest of the five field-parity fixes: ckptinfo is a bare string, so no
capnp struct is needed -- mirrors WAL_QUEUE_CTRL's existing NUL-terminated-
string-after-header wire shape instead. Adoption is unconditional (not
gated on a state transition), matching bug-1690's precedent: a checkpoint
report never changes job.state. restore_job_ckpt_state() releases the
prior value via dereference() (the correct RTTI verb for an xstring), not
free().

Found by an /xce-audit-invariants controller pass (field-complete apply
parity dimension) -- the last of five sibling gaps found together
(bug-1678, bug-1687, bug-1689, bug-1690, this one). 7 new tests in
test_wal_job_ckpt_fact.c; RED/GREEN independently verified (5 genuine
pre-fix failures, 2 backward-compat tests correctly passed unchanged).
Full controller suite: 1167 tests, 2 failures -- both the pre-existing,
unrelated bug-1561 overlayfs sandbox flake. No ASAN findings.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg

src/controller/meson.build

@@ -173,6 +173,7 @@ controller_unit_test_src = [
   'testsuite/test_wal_job_start_fact.c',
   'testsuite/test_wal_job_finish_fact.c',
   'testsuite/test_wal_job_rusage_fact.c',
+  'testsuite/test_wal_job_ckpt_fact.c',
   'testsuite/test_job_dispatch_fail_count.c',
   'testsuite/test_apply_reapply_idempotency.c',
   'testsuite/test_apply_correctness_verification.c',