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/testsuite/controller_test.c

@@ -12,6 +12,7 @@ extern void test_persist_checkpoint(void);
 extern void test_persist_recovery(void);
 extern void test_hooks(void);
 extern void test_persist_hooks(void);
+extern void test_gc_retention(void);
 
 void setUp(void) { return; }
 
@@ -93,6 +94,7 @@ int main(int argc, char* argv[])
     TEST(test_persist_recovery);
     TEST(test_hooks);
     TEST(test_persist_hooks);
+    TEST(test_gc_retention);
 
     return UNITY_END();
 }