fix(ha-repl-hardening): n_peers count and single-node safe_lsn (task 6.5)

- Fix ha_start_election_async: ha->n_peers = n_other (other peers),
  not candidates_count (total including self). The standby_converged[]
  array is indexed by peer index, so the bound must be n_other.
- Fix ha_compute_safe_committed_lsn: return self_lsn for n_peers==0
  (prevents WAL truncation on single-node re-election).
- Integration test: 3-node cluster reaches CTRL_PRIMARY via majority
  convergence within ~20s; no degraded-mode WARN in logs.

src/controller/ha.c

@@ -3041,23 +3041,29 @@ void ha_start_election_async(ha_state_t *ha, cluster_t *c,
         raft_init(&ha->raft, &rcfg);
     }
 
-    /* Step 1: store peer count for ha_repl_ack_received quorum calculation */
-    ha->n_peers = n_peers;
-
-    /* Step 2: transition to CTRL_ELECTING before firing RPCs.
+    /* Step 1: transition to CTRL_ELECTING before firing RPCs.
      * raft_start_election fires on_become_leader / on_send_vote_request below;
      * the role must already be ELECTING so ha_dispatch_gate() admits vote RPCs. */
     atomic_store_explicit(&ha->role, (int32_t)CTRL_ELECTING, memory_order_release);
     INFO("HA FSM: → CTRL_ELECTING (async, n_peers=%d)", n_peers);
 
-    /* Step 3: pre-allocate ha_elect_ctx_t so raft_cb_send_vote_request can
+    /* Step 2: pre-allocate ha_elect_ctx_t so raft_cb_send_vote_request can
      * read my_tag/my_lsn from it when building the capnp VoteRequest payload.
      * Vote counting is now owned by the raft library; this ctx is only a
      * metadata carrier for tag/lsn.  For multi-node elections it is kept alive
      * until all per-peer vote_ep_ctx_t endpoints finish (iomgr lifetime).
      * For single-node, raft_start_election fires on_become_leader immediately
-     * so no ctx reads occur after return. */
+     * so no ctx reads occur after return.
+     *
+     * n_other = other-node count (self excluded).  standby_converged[] is
+     * indexed by votes_idx (relative, 0 .. n_other-1), so ha->n_peers must
+     * store n_other, NOT the total candidates_count.  Both
+     * ha_check_majority_convergence and ha_repl_ack_received read ha->n_peers
+     * as "number of OTHER nodes". */
     int n_other = (n_peers > 1) ? n_peers - 1 : 0;
+
+    /* Store other-peer count for quorum calculations */
+    ha->n_peers = n_other;
     ha_elect_ctx_t *ctx = calloc(1, sizeof(*ctx) +
                                   (size_t)n_other * sizeof(ctx->votes[0]));
     if (!ctx) {