The update_batch_nios handler looped serially across all entries (5010
for a 2505-link topology). On started nodes each entry does uBridge I/O,
so the serial loop added orders of magnitude to the fan-out wall time.
Group entries by node_id before dispatching. Different nodes talk to
their own uBridge process (AF_UNIX socket) and are fully independent, so
their updates run in parallel via asyncio.gather. Per-node entries are
still serial (respecting the per-node uBridge command lock).