refactor(ha-repl-hardening): standardize n_peers = total cluster size including self

- ha_state_t.n_peers now matches raft_config_t.n_peers convention:
  total cluster size (including self), consistently across both layers
- ha_start_election_async: ha->n_peers = n_peers (candidates_count)
- Convergence/counting loops: i < n_peers - 1 (skip self for standby arrays)
- Single-node: n_peers=1, threshold=0 (same behavior, clear semantics)
- ha_elect_ctx_t.n_peers unchanged (separate field for vote array sizing)
- Tests: 3-node n_peers=2->3, single-node 0->1, loop bounds updated

E2E test 6.7: 3-node cluster lifecycle verified
- Initial: ctrl-1 PRIMARY via majority convergence, no degraded mode
- Kill primary: RDHUP detected on both standbys, ctrl-2 elected at term=4
- No data loss: config tag preserved across failover
- Full ASAN build green

src/controller/ha.h

@@ -176,7 +176,7 @@ typedef struct {
     void            *client_ssl_ctx;    /* SSL_CTX* (TLS_client_method); created in myapp_run */
     int              phase2_done;       /* 1 after cluster_init_phase2 runs */
     _Atomic int      repl_ack_count;    /* count of standbys that ACKed in COMMITTING state */
-    int              n_peers;           /* total candidate count (excluding self) */
+    int              n_peers;           /* total cluster size (including self), matches raft convention */
     /* ── New fields added by raft-library spec (Task 6.1) ──────── */
     raft_state_t     raft;              /* Raft consensus state (wired in task 6.2) */
     /* ── New fields added by raft-library spec (Task 6.2) ──────── */
@@ -389,8 +389,8 @@ void ha_process_standby_state(ha_state_t *ha, int peer_idx,
  * ha_check_majority_convergence: count converged standbys and promote to
  * CTRL_PRIMARY when the threshold is met while role == CTRL_PRIMARY_COMMITTING.
  *
- * Threshold: ha->n_peers (all standbys must converge).
- * Single-node (n_peers=0): threshold=0 → satisfied immediately.
+ * Threshold: ha->n_peers - 1 (all standbys must converge).
+ * Single-node (n_peers=1): threshold=0 → satisfied immediately.
  *
  * Double-promotion is prevented by the single-threaded vtask context; a role
  * check (CTRL_PRIMARY_COMMITTING) is sufficient.