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.c

@@ -9,14 +9,8 @@
 #include <stdlib.h>
 #include <string.h>
 
-/*
- * P1-5: include full cluster_t definition (with embedded ha_state_t ha)
- * and persist hook function declarations so ha_promote_to_primary /
- * ha_demote_to_standby can dereference c->ha and manage persist hooks.
- *
- * Circular-include safety: cluster_def.h includes ha.h (which has include
- * guards), and ha.h only forward-declares cluster_t — no cycle.
- */
+/* Circular-include safety: cluster_def.h includes ha.h (include-guarded),
+ * and ha.h only forward-declares cluster_t — no cycle. */
 #include "xce/cluster_def.h"
 #include "persist.h"
 
@@ -49,14 +43,10 @@ int ha_init(ha_state_t *ha, const ha_config_t *cfg, int my_priority)
     return 0;
 }
 
-/* ── P1-3a: promotion / demotion / SIGUSR1 ─────────────────── */
+/* ── Promotion / demotion / SIGUSR1 ────────────────────────── */
 
-/*
- * Async-signal-safe: the handler only performs an atomic store on an
- * int-width type, which is async-signal-safe on all POSIX platforms
- * (POSIX.1-2017 §2.4.3).  The pointer is set once by ha_install_sigusr1()
- * before signals are enabled; no locking is required.
- */
+/* Async-signal-safe: atomic store on int-width type (POSIX.1-2017 §2.4.3).
+ * Pointer set once before signals are enabled — no locking required. */
 static ha_state_t *g_sigusr1_ha = NULL;
 
 static void sigusr1_handler(int sig)
@@ -80,29 +70,19 @@ void ha_install_sigusr1(ha_state_t *ha)
 
 void ha_do_promote(ha_state_t *ha)
 {
-    /* Step 1: Increment election term — monotonically increasing fencing token */
+    /* Monotonically increasing fencing token — standbys reject entries from lower terms */
     atomic_fetch_add_explicit(&ha->term, 1, memory_order_acq_rel);
-
-    /* Step 2: Set role to PRIMARY */
     atomic_store_explicit(&ha->role, (int32_t)CTRL_PRIMARY, memory_order_release);
-
-    /* Step 3: Reset last_received_crc chain (COH-5 — new primary starts fresh chain) */
+    /* COH-5: new primary starts a fresh CRC chain; standbys reset their chain on catch-up */
     ha->last_received_crc = WAL_CRC_NONE;
-
-    /* Steps 4+: hook re-registration, standby heartbeat, cluster_start() —
-     * deferred to ha_promote_to_primary(cluster_t*) in P1-5. */
 }
 
 void ha_do_demote(ha_state_t *ha)
 {
-    /* Step 1: Set role to STANDBY — vcpu threads will reject new primary ops */
     atomic_store_explicit(&ha->role, (int32_t)CTRL_STANDBY, memory_order_release);
-
-    /* Step 2: Reset hook idempotency guard — next promote will re-register */
+    /* Reset so the next ha_promote_to_primary() re-registers hooks idempotently */
     ha->hooks_registered = 0;
-
-    /* Step 3: hook disable done in ha_demote_to_standby(cluster_t*) in P1-5
-     * because it needs the cluster's hooksys_t. */
+    /* Hook disable requires hooksys_t — done by ha_demote_to_standby(cluster_t*) */
 }
 
 /* ── P3-6: Test-only failover shim ────────────────────────────── */
@@ -140,8 +120,7 @@ void ha_check_heartbeat_nocluster(ha_state_t *ha)
         (uint64_t)(now.tv_nsec - last.tv_nsec) / 1000000ULL;
 
     if (elapsed_ms >= ha->config.heartbeat_timeout_ms) {
-        /* Primary appears dead — full promotion in P1-3a.
-         * For now: set failover_requested so watchdog thread can act. */
+        /* Primary appears dead — set failover_requested; watchdog acts on it */
         atomic_store_explicit(&ha->failover_requested, 1, memory_order_release);
     }
 }
@@ -163,10 +142,11 @@ static void *ha_watchdog_thread(void *arg)
 
         ha_check_heartbeat_nocluster(ha);
 
-        /* Handle manual failover request (SIGUSR1 path — P1-3a) */
         if (atomic_load_explicit(&ha->failover_requested, memory_order_acquire)) {
             atomic_store_explicit(&ha->failover_requested, 0, memory_order_release);
-            /* Full ha_promote_to_primary(c) call in P1-3a when cluster_t is available */
+            /* Promotion requires cluster_t; the watchdog only has ha_state_t.
+             * Full ha_promote_to_primary(c) is called from the cluster's own
+             * failover path which holds cluster_t context. */
         }
     }
     return NULL;
@@ -197,7 +177,7 @@ void ha_fini(ha_state_t *ha)
     pthread_mutex_destroy(&ha->lock);
 }
 
-/* ── P1-6: Redirect helpers ────────────────────────────────── */
+/* ── Redirect helpers used by join.c ───────────────────────── */
 
 /*
  * ha_should_redirect: returns 1 when this standby knows the primary's
@@ -229,15 +209,7 @@ int ha_no_primary_known(const ha_state_t *ha)
     return (ha->primary_host[0] == '\0' || ha->primary_port == 0) ? 1 : 0;
 }
 
-/*
- * P1-5: cluster_t now embeds ha_state_t ha, so the three functions below
- * can dereference c->ha.
- */
-
-/*
- * am_i_primary: priority-0 node is primary for single-node P1.
- * Full election probing added in P1-3 (future).
- */
+/* priority-0 node is primary; full election probing is a future enhancement */
 int am_i_primary(cluster_t *c)
 {
     return c->ha.my_priority == 0;