feat: wire HA candidate-list reconnect into worker+scheduler join paths

Worker (xcecd):
- Add xce_cluster_config_t cluster_cfg field to controller_connector_ctx_t
  (controller_connector.h includes xce/cluster.h)
- Populate cc_ctx->cluster_cfg from parsed cluster_cfg after xcecd_start()
  sets cc_ctx (main.c, removes the P3 placeholder)
- Replace xcecd_join_cluster() single-shot connect with a candidate-iterating
  loop: tries each candidate, calls xcecli_stop() before the next attempt,
  falls back to the original single-host path when candidates_count == 0

Scheduler (xce_scheduler):
- Add xce_cluster_config_t cluster_cfg field to xce_scheduler_t
  (scheduler.h includes xce/cluster.h)
- Add sched_set_cluster_cfg() setter
- Replace the hardcoded xcecli_start/xcecli_join pair in sched_init() with
  the same candidate-iterating pattern; original single-host path preserved
- main.c calls sched_set_cluster_cfg() before sched_init() when --peers given
  (removes the P3 placeholder comment)

checking_connection() already calls xcecd_join_cluster() on retry, giving
the worker free reconnect-after-failover behaviour for no additional code.

src/scheduler/include/scheduler.h

@@ -15,6 +15,7 @@
 #include "kvmgr.h"
 #include "mylock.h"
 #include "plugin.h"
+#include "xce/cluster.h"
 #include "xce/types.h"
 #include "xlist.h"
 #include "xvec.h"
@@ -88,10 +89,12 @@ typedef struct {
     _Atomic int        eval_in_progress;       /* 1 while eval_select loop runs */
     _Atomic int        staging_merge_complete; /* 1 after staging merge completes */
     select_eval_ctx_t* sel_eval_ctx;           /* session-level memoisation context; NULL between sessions */
+    xce_cluster_config_t cluster_cfg;          /* HA candidate list — empty means single-host mode */
 } xce_scheduler_t;
 
 extern int  sched_init(xce_scheduler_t*, xce_client_config_t*,
                        xce_request_handler_t);
+extern void sched_set_cluster_cfg(xce_scheduler_t*, const xce_cluster_config_t*);
 extern void sched_fini(xce_scheduler_t*);
 extern int  sched_init_config(xce_scheduler_t* sched, xce_conf4sched_t*);
 extern int  sched_update(xce_scheduler_t*);