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/include/xce/types.h

@@ -172,6 +172,7 @@ typedef struct {
 #define XCE_WORKER_LOCKEDW (0x00040000)
 #define XCE_WORKER_BUSY    (0x00080000)
 #define XCE_WORKER_CLOSED  (0x00100000)
+#define XCE_WORKER_HIDDEN  (0x00200000) /* Admin explicitly removed — invisible but not yet cleaned up */
 
 #define IS_WORKER_UNAVAIL(status) (((status) & XCE_WORKER_UNAVAIL) != 0)
 #define IS_WORKER_LOCKEDU(status) (((status) & XCE_WORKER_LOCKEDU) != 0)
@@ -353,7 +354,8 @@ typedef struct {
 
 #define JOB_STAT_PDONE (0x80)  /**< Post job process done successfully */
 #define JOB_STAT_PERR  (0x100) /**< Post job process has error */
-#define JOB_STAT_WAIT  (0x200) /**< Chunk job waiting its turn to exec */
+#define JOB_STAT_WAIT      (0x200) /**< Chunk job waiting its turn to exec */
+#define JOB_STAT_TRANSIENT (0x400) /**< Job state uncertain after failover — awaiting worker re-join */
 #define JOB_STAT_RUNKWN                                                        \
     0x8000 /* Flag : Job status is UNKWN caused by losting contact with remote \
               cluster */
@@ -381,6 +383,7 @@ typedef struct {
 #define QUEUE_STAT_DISC        0x10 /**< Remote queue status is disconnected. */
 #define QUEUE_STAT_FAIRSHARE_DEFAULT                                           \
     0x20 /**< Fairshare queue with null user share defined*/
+#define QUEUE_STAT_HIDDEN (0x0100) /**< Admin deleted — invisible but not yet cleaned up */
 
 /* FIXME: improve following later */
 #define JOB_IS_PEND(state)   (((state) & JOB_STAT_PEND) != 0)