fix(controller): scan standby_endpoints by capacity, not count (bug-1474)

Root cause of the "spurious duplicate reconnect" (angle a): zarray_del()
NULLs the slot and decrements count without compacting, so a live entry
can sit at any index < caps. Every standby_endpoints loop bounded by
`count` missed it. Live evidence (--config-upgrade-wal-trim soak,
2026-09-26): ctrl-2 won, promoted vote eps [ctrl-1 (dead), ctrl-3]; the
dead one's removal left [NULL, ctrl-3] with count 1, so the primary

- stopped heartbeating ctrl-3 (primary_heartbeat_fn's send loop),
- re-dialed it ("peer[2] not connected -- initiating outbound connect"
  9s later, for a peer that never dropped) and registered a second,
  "late-joiner" channel that burned a MAX_STANDBYS repl slot,
- could not evict the stale channel (on_standby_connected's dedup scan),
- and skipped it in config pushes and the ctrlhello broadcast, where a
  hole below count also handed endpoint_ref() a NULL.

The re-dial landed in the freed slot 0, which is what made ctrl-3 visible
again -- the "spurious" reconnect was masking the lost heartbeats.

All index scans of standby_endpoints (ha.c heartbeat, sweep, dedup, two
removals; cluster.c removal and ctrlhello; config_repl.c push collection)
now run to `caps` and skip NULL slots; the push collects exactly the
non-NULL entries. `count` stays where it means "how many connected".

New test_reconnect_dedups_channel_behind_deleted_slot (RED before: the
stale channel behind the hole was never evicted). Controller suite 1241/0.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/controller/cluster.c

@@ -88,7 +88,7 @@ static void handle_endpoint_error(endpoint_t* ep)
             if (c->ha.standby_endpoints) {
                 pthread_mutex_lock(&c->ha.lock);
                 for (int idx = 0;
-                     idx < c->ha.standby_endpoints->count; idx++) {
+                     idx < c->ha.standby_endpoints->caps; idx++) {
                     endpoint_t *se = (endpoint_t *)zarray_get(
                         c->ha.standby_endpoints, idx);
                     if (se == ep) {
@@ -2424,7 +2424,8 @@ void ha_send_ctrlhello_to_all_standbys(cluster_t *c)
     char    *my_name   = c->myconf ? c->myconf->name : "";
 
     pthread_mutex_lock(&ha->lock);
-    int n = ha->standby_endpoints->count;
+    /* bug-1474: zarray_del leaves holes, so scan caps, not count. */
+    int n = ha->standby_endpoints->caps;
     for (int i = 0; i < n; i++) {
         endpoint_t *ep = (endpoint_t *)zarray_get(ha->standby_endpoints, i);
         if (!ep) continue;