feat(libxce-failover): endpoint rotator — hint-first target selection + master rotation; keep api_ctx_t decls private (task 2.2)

src/include/xce/client_failover.h

@@ -46,4 +46,10 @@ typedef enum {
  */
 xce_fo_action_t xce_failover_classify(int status, bool is_write, int attempt);
 
+/*
+ * EndpointRotator (xce_failover_next_target / xce_failover_rotate) operates on
+ * the private api_ctx_t and is therefore declared in the internal header
+ * src/lib/client_internal.h, not in this public surface.
+ */
+
 #endif /* XCE_CLIENT_FAILOVER_H */