fix: memory pool alloc mismatch in tests, db_sync SEND_DATA buffer overflow

- test_etcp_router_reconnect, test_etcp_connect, test_stcp_traffic:
  replace u_calloc with memory_pool_alloc for TOPO_SOCKMETA4/TOPO_ADDR4
- db_sync.c: fix req[14] -> req[15] — SEND_DATA request header is 15 bytes
  (type:1 + from:4 + count:2 + vp:4 + want_from:4)

src/utun_instance.h

@@ -114,7 +114,7 @@ struct UTUN_INSTANCE {
     struct ll_queue* tcp_connections;    // indexed by node_id, data=tcp_conn_entry [TCP-only]
     
     // Callback chain for new ETCP connections
-    struct etcp_cbk_entry* new_conn_cbks;
+    struct etcp_inst_cbk_entry* new_conn_cbks;
     struct etcp_status_cbk_entry* conn_status_cbks;  // instance-level: NEW/UP/DOWN/DELETE
     void* test_user_ptr;             // Generic user pointer (used by tests)