Replace the default asyncio executor (capped at ~32 threads) with a
dedicated ThreadPoolExecutor sized for large-topology parallelism.
When 500 nodes each call _connect_nio, up to 500 OS threads can now
send blocking ubridge commands in parallel — no longer serialised by
either the event loop or a small thread pool.
- ubridge_hypervisor: module-level _ubridge_sync_pool (max_workers=500)
- docker_vm._connect_nio: dispatches to the dedicated pool instead of
the default executor
b155980402