enum PoaDispatchExecutor

Where servant dispatch runs for objects activated on a POA.

• inlineOnTransportThread: default — SHM ring / transport thread (lowest latency; not MainActor-safe).
• main: SHM ring fire-and-forgets the whole request onto DispatchQueue.main (no Asio hop). Servant + reply run on main.
• queue: same for a custom serial DispatchQueue (required for FIFO SHM reply matching).
• loop: same for a thread that is not a DispatchQueue at all — see PoaExecutor.

Avoid blocking main on NPRPC work that itself waits on main.