Election carrier (ha_elect_ctx_t) is never freed today -- each election/PreVote
round deliberately leaks the previous one because per-peer vote endpoints hold a
bare pointer into it and tear down asynchronously, so a naive free risks UAF.
Requirements scope this to a memory-lifetime-only fix: free once no vote
endpoint can reference it, never free while one might, uniformly across the
real-election and PreVote paths. No election/failover semantic change.
