Owns long-lived Gun connections on behalf of the Gun adapter.

This process exists so named Gun-backed channels are no longer tied to the lifecycle of the process that happened to call GRPC.Stub.connect/2.

Without this wrapper, GRPC.Client.Connection would need to own Gun directly or understand Gun-specific owner messages. Keeping Gun ownership in this adapter-local process preserves a clean adapter boundary while ensuring the underlying Gun connection survives short-lived callers.

Request-specific Gun messages are routed to per-stream response processes, so this process only needs to manage connection-level lifecycle and stream bookkeeping.