Introduce an object-oriented Go mirror of the Nim Messaging API over the C bindings

[igor-sirotin]

Sub-task of #106.

Provide a high-level, idiomatic Go API that mirrors logos-delivery's Nim MessagingClient, implemented over the liblogosdelivery C bindings (internal/ffi) rather than exposing the raw FFI.

Scope

• MessagingClient: New / Start / Stop / Close, Subscribe / Unsubscribe, Send(ctx, Envelope) (RequestID, error).
• A unified Events() <-chan Event with a sealed Event interface (MessageReceived / Sent / Propagated / Error, ConnectionStatus); non-blocking delivery.
• Typed Envelope / Config; event decoding that matches the library's wire format.
• Runnable example + unit tests.

Introduced by #117 (stacked on the single-library migration).