fix(controller): checkpoints keep a job's exit code (bug-1867)

The capnp Job struct -- the checkpoint/entity-snapshot form of xce_job_t --
had no exit-info field, so capnp_encode_job() never wrote job.exit_info
and a job restored from a snapshot (instead of WAL replay) came back with
exit_info NULL, for DONE/EXIT and failed jobs alike. bug-1689/bug-1754
made the WAL facts carry the code; anything older than the last
checkpoint's WAL trim still lost it.

- controller.capnp: struct JobExitInfo $C.mapname("job_exitinfo_t")
  { exitcode }, and Job.exitInfo @12 $C.mapname("exit_info"). The
  generator now emits encode/decode/free for it inside encode_Job /
  decode_Job / free_Job; restore_job()'s create_job() already takes
  exit_info via MOVE_PTR. Null stays null (no fabricated exit 0). A new
  ordinal, so older snapshots/recordings decode with exit_info NULL.

New test_snapent_checkpoint_exit_info_roundtrip (RED before): exit codes
0 and 137 and a NULL survive both the raw checkpoint frames and a real
persist_load_snapshot restore. Full /xce-build all (asan): 111 meson
suites, 0 failures; all Go packages pass.

Not live-verified: the --force-snapshot HA soak cannot reach its snapshot
step because of a pre-existing checkpoint-marker replication stall,
reproduced on the baseline without this change (bug-1870).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/schema/controller.capnp

@@ -333,6 +333,14 @@ struct Job $C.mapname("xce_job_t"){
   pidinfo        @9: JobPIDInfo;
   pendreason     @10: JobPendReason $C.mapname("pend_reasons");
   transientSince @11: UInt64 = 0 $C.mapname("transient_since");  # epoch seconds; set when job enters JOB_STAT_TRANSIENT
+  exitInfo       @12: JobExitInfo $C.mapname("exit_info");  # bug-1867: null until the job's exit code is known
+}
+
+# bug-1867: job_exitinfo_t, so a checkpointed Job keeps its exit code. The
+# WAL facts (WAL_JOB_FINISH / WAL_JOB_FAIL) still carry it as a native
+# int32_t trailer; this is the snapshot/entity form.
+struct JobExitInfo $C.mapname("job_exitinfo_t") {
+  exitcode @0: Int32;
 }
 
 struct SchedJob $C.mapname("xce_job_t"){