feat: Wire event dispatcher for ctx.events.dispatch

[doubleailes]

Summary

Closes the third and final piece of the async-handler refactor — events.

• New girolle/src/events.rs — EventDispatcherCore owns a publish channel and a DashSet<String> cache of declared {source}.events topic exchanges (durable: true). Exchanges are declared lazily the first time a given source emits, so a single dispatcher can fan out events for multiple sources.
• EventDispatcher now holds Option<Arc<EventDispatcherCore>> plus the parent delivery's FieldTable. pub async fn dispatch<T: Serialize>(source_service, event_type, &payload) serializes the payload to JSON and publishes with persistent delivery, JSON content-type, and event_type as the routing key. nameko.AMQP_URI is stamped onto headers and the parent's nameko.call_id_stack (when present) is propagated through.
• RpcService startup calls EventDispatcherCore::new(&conn, conf, id) once and wires the resulting dispatcher into SharedData. Per delivery the consumer does with_parent_headers(inbound_headers.clone()) for both rpc and events.
• examples/src/event_emitter.rs — a service that emits a user_created event on users.events from inside a handler.

Acceptance

With this merged, a handler can:

• Receive an RpcContext carrying inbound metadata
• Call other services via ctx.rpc.call(...)
• Emit Nameko-compatible events via ctx.events.dispatch(...)

…which closes out the original async-handler issue's acceptance criteria.

Known limitations (deferred)

• No publisher confirms — dispatch returns once the publish has been handed to the broker, no broker-side ack.
• Outbound call_id_stack propagated as-is from the parent (matches the RPC caller's behavior; see feat: Wire in-service RPC core for ctx.rpc.call #156).
• No event consumer support yet — that's a separate piece of work.

Test plan

• `cargo build --workspace`
• `cargo build --examples` (incl. new `event_emitter`)
• `cargo test --workspace --lib` (14 passed)
• `cargo test --workspace --doc` (30 + 1 passed)
• `cargo clippy --workspace --all-targets`
• Smoke test: run `event_emitter`, call `users.create_user`, observe a message on `users.events` with routing key `user_created` (e.g. via the RabbitMQ management UI or a Nameko subscriber).

🤖 Generated with Claude Code

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