create_batch_nios bound NIOs in a serial for-loop, so during project open
every builtin L2 node (ethernet_switch/hub/cloud/nat) started its uBridge
one at a time (~0.5s each for fork + AF_UNIX connect). Group entries by
node and bind in parallel with asyncio.gather — mirroring update_batch_nios
— so independent uBridge processes start concurrently. Within a node,
entries stay serial to respect the per-node uBridge command lock.
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