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/persist.h

@@ -163,3 +163,10 @@ int persist_queue_hook_fun(int type, int situation, int detail,
                            void* cls, va_list ap);
 int persist_worker_hook_fun(int type, int situation, int detail,
                             void* cls, va_list ap);
+
+/* GC retention predicates (P0-5a / P0-8 / P0-13) */
+#include <stdbool.h>
+#include "xce/types.h"
+bool job_is_terminal(const job_t* j);
+bool job_should_retain(const job_t* j, const persist_config_t* cfg);
+bool worker_should_retain(const xce_worker_t* w, const persist_config_t* cfg);