feat(ha-election): per-priority failover election stagger + prevote dispatch-gate

Desync FAILOVER re-elections by node priority (ha_election_stagger_ms /
ha_failover_stagger_fn) mirroring the startup stagger, with seed jitter and
async prevote start; allow prevote in voter roles through the dispatch gate.
Adds HA role and vote-gate tests (stagger desync by priority, prevote on dead
primary, stickiness-lapse grants).

Pre-existing uncommitted work, committed to establish a clean baseline before
ha-commit-durability implementation.

src/controller/cluster.c

@@ -523,6 +523,7 @@ static int myreq_handler(vtask_t* task)
                     if (role == CTRL_PRIMARY)
                         deny_reason = "primary";
                     else if (role == CTRL_STANDBY &&
+                             c->ha.primary_ctrl_fd >= 0 &&
                              ha_heartbeat_is_fresh(&c->ha))
                         deny_reason = "fresh_heartbeat";
                     else if (atomic_load_explicit(&c->ha.in_catch_up,