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/meson.build

@@ -75,6 +75,7 @@ controller_unit_test_src = [
   'testsuite/test_persist_recovery.c',
   'testsuite/test_hooks.c',
   'testsuite/test_persist_hooks.c',
+  'testsuite/test_gc_retention.c',
   'persist.c',
 ]
 
@@ -95,5 +96,5 @@ controller_unit_test = executable('controller_unit_test',
 
 test('controller test', controller_unit_test,
      suite: 'controller',
-     timeout: 60)
+     timeout: 120)