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/worker.c

@@ -1,7 +1,10 @@
 #include <errno.h>
+#include <stdbool.h>
 #include <stdlib.h>
+#include <time.h>
 
 #include "cluster.h"
+#include "persist.h"
 #include "wobj.h"
 #include "worker.h"
 #include "xce/cluster_def.h"
@@ -519,6 +522,8 @@ int mark_worker_unavail(cluster_t* c, worker_t* w)
                   HOOK_DETAIL_WORKER_DEAD, c, w);
 
     w->host.status |= XCE_WORKER_UNAVAIL;
+    if (w->host.dead_since == 0)
+        w->host.dead_since = time(NULL);
 
     xce_run_hooks(&(c->hooksys), XCE_CONTROLLER_HOOK_WORKER, HOOK_AFTER_EVENT,
                   HOOK_DETAIL_WORKER_DEAD, c, w);
@@ -658,3 +663,7 @@ int worker_from_capnp(cluster_t* c, const void* capnp_payload, size_t len)
     (void)len;
     return 0;
 }
+
+/* worker_should_retain lives in persist.c —
+ * it only depends on xce_worker_t fields and persist_config_t,
+ * and must be accessible from the unit test binary which links persist.c. */