refactor: simplify ha.c comments and extract HA config constants

- Add HA_DEFAULT_* constants to ha.h; use them in cluster_init()
- Strip stale task-label comments (P1-3a, P1-5, P1-6, P3-6 refs)
- Remove self-documenting 'Step N:' annotations from ha_do_promote/demote
- Rewrite stale forward-reference notes as current WHY comments
- Remove resolved TODO P0-8b from cluster.c

src/controller/ha.h

@@ -35,6 +35,14 @@ typedef enum {
 } ctrl_role_t;
 
 /* ── HA configuration ──────────────────────────────────────── */
+
+/* Default values used by cluster_init(). Tuned for a LAN with ≤5ms RTT. */
+#define HA_DEFAULT_HEARTBEAT_INTERVAL_MS    3000
+#define HA_DEFAULT_HEARTBEAT_TIMEOUT_MS     10000
+#define HA_DEFAULT_ELECTION_TIMEOUT_MS      2000
+#define HA_DEFAULT_PROBE_CONNECT_TIMEOUT_MS 500
+#define HA_DEFAULT_MAX_PROMOTION_RETRIES    5
+
 typedef struct {
     uint32_t heartbeat_interval_ms;    /* Primary heartbeat send interval (min 2000 ms) */
     uint32_t heartbeat_timeout_ms;     /* Standby timeout before promoting (>= interval) */