feat(ha-commit-durability): add --wal-sync-ms flush-cadence override (task 1.1)

src/include/myconf.h

@@ -36,6 +36,7 @@ typedef struct {
     int                  conf_stale;             /* 1 when saved config older than conf_stale_timeout */
     int                  vcpus;                 /* Req 4.3: --vcpus CLI value; 0 = use config default */
     uint32_t             sync_timeout_ms;       /* --sync-timeout-ms; 0 = use persist default */
+    uint32_t             wal_sync_ms;           /* --wal-sync-ms; 0 = use persist default */
 } myconf_t;
 
 extern cluster_config_t* get_default_conf(void*);

src/include/persist.h

@@ -42,6 +42,24 @@ static inline persist_config_t persist_config_defaults(void)
     return c;
 }
 
+/*
+ * persist_config_set_wal_sync_ms — apply the --wal-sync-ms flush-cadence
+ * override (ha-commit-durability Req 3.1).
+ *
+ * Single override point shared by both persist_config_defaults() callers
+ * (cluster.c phase2 and myapp.c step 6), mirroring the --sync-timeout-ms
+ * convention: override_ms == 0 means "flag not passed" and leaves the
+ * default flush cadence untouched (Req 3.3 — the seam changes timing
+ * controllability only, not the durability contract). A positive value
+ * sets the cadence to exactly that, applied before persistence starts.
+ */
+static inline void persist_config_set_wal_sync_ms(persist_config_t *c,
+                                                  uint32_t          override_ms)
+{
+    if (override_ms > 0)
+        c->wal_sync_ms = override_ms;
+}
+
 /* ============================================================
  * Key→index mapping for checkpoint serialization (Part 3.8)
  * ============================================================ */