feat: JOB_STAT_TRANSIENT, job_should_retain, worker_should_retain, dead_since (P0-5a, P0-8, P0-13)

Add terminal-state constants and GC retention predicates for the
checkpoint xzarray_thaw() GC pass:

- types.h: add JOB_STAT_TRANSIENT (0x400), XCE_WORKER_HIDDEN (0x00200000),
  QUEUE_STAT_HIDDEN (0x0100)
- persist.c: implement job_is_terminal(), job_should_retain(),
  worker_should_retain() — placed here so the unit test binary
  (which links only persist.c) can access them without RTTI deps
- persist.h: declare the three GC predicates; include xce/types.h
- worker.c: set dead_since = time(NULL) in mark_worker_unavail()
  when dead_since is not already set
- cluster.c: add TODO P0-8b marker on zarray_del endpoint cleanup
- meson.build: register test_gc_retention.c; raise test timeout 60→120s
- testsuite/test_gc_retention.c: 19 tests covering job_is_terminal,
  job_should_retain (TRANSIENT TTL, terminal TTL, non-terminal always
  retained), and worker_should_retain (HIDDEN/UNAVAIL/healthy/zero
  dead_since); all pass (63 total, 0 failures)

src/controller/job.c

@@ -1,9 +1,11 @@
 #include <errno.h>
 #include <signal.h>
+#include <stdbool.h>
 #include <time.h>
 
 #include "cluster.h"
 #include "jobj.h"
+#include "persist.h"
 #include "preason.h"
 #include "xce/cluster_def.h"
 #include "xce/errnox.h"
@@ -1040,3 +1042,7 @@ int do_clr_preason(cluster_t* c, endpoint_t* ep, xce_request_t* request)
 
     return 0;
 }
+
+/* job_is_terminal and job_should_retain live in persist.c —
+ * they only depend on xce_job_t fields and persist_config_t,
+ * and must be accessible from the unit test binary which links persist.c. */