Optional host-owned bounded concurrency, with no startup/recovery work.

Add {Opsmaru.Incus.ClientSupervisor, name: MyHost.Clients, max_children: 8} to the host's tree. Calls at capacity fail immediately; no work queue is kept. Stopping/restarting it affects only local calls, never host/Incus lifecycle. Losing a local worker cannot establish backend cancellation or permit replay.