feat(raft-drive-async): SlowLane bounded executor for long blocking I/O (task 1.2)

Dedicated-pthread pool isolated from the vtask pool; non-blocking slowlane_submit
(0 enqueued / -EEXIST deduped / -EAGAIN cap-full-deferred), admission cap on
running tasks, per-key in-flight dedup, run-exactly-once, drain+join fini. Wired
into controller lifecycle (dormant; consumers land in 2.x/3.x). 7 unit tests,
mutation-verified cap+dedup; full suite 194/0 green, 15x ASAN.

src/include/xce/cluster_def.h

@@ -20,6 +20,7 @@
 /* ha.h forward-declares cluster_t; including it here is safe (no circular dep) */
 #include "commit.h"
 #include "ha.h"
+#include "slowlane.h"
 #include "xce/types.h"
 #include "xlist.h"
 #include "zarray.h"
@@ -123,6 +124,7 @@ typedef struct cluster_s {
     persist_t           persist;   /* Persistence module — P0-F */
     ha_state_t          ha;        /* HA role management — P1-5 */
     commit_engine_t     commit;    /* Per-entry commit engine — ha-commit-engine 5.3 */
+    slowlane_t          slowlane;  /* Long-blocking-IO offload lane — raft-drive-async 1.2 */
     _Atomic int         restart_requested;           /* set by OP_upgradeConfig handler */
     pthread_t           heartbeat_listener_tid;      /* pre-Phase-2 SSL listener thread */
     _Atomic int         heartbeat_listener_running;