The _connect_nio thread-pool optimisation (send_batch_sync) targeted
node-start performance, but start_all already runs at concurrency=3
(by design, to avoid overwhelming the host). It also introduced a
Python 3.13 incompatibility (trsock.setblocking forbidden) that
prevented docker nodes from starting. Since node-start is not the
target of this branch (project-open link creation is), revert to the
simple per-command async _ubridge_send.
The project-open batch NIO dispatch (create_batch_nios) is unaffected —
it never called _connect_nio (nodes aren't started during open).