feat: Add event subscriptions and CLI sender

[doubleailes]

Summary

Tooling + lib changes that make the new RPC and events features testable locally without the RabbitMQ management UI.

Library

• New EventHandler type alias mirroring RpcHandler: `Arc<dyn Fn(RpcContext, Value) -> BoxFuture<GirolleResult<()>> + Send + Sync>`.
• `RpcService::subscribe(source_service, event_type, handler)` registers an async event handler. A service may now consist of RPC methods, event subscriptions, or both. The empty-service panic now triggers only when both are empty.
• New `queue::create_event_consumer_channel` declares the source's `{source}.events` topic exchange (idempotent, durable), declares a durable consumer queue `evt--<event_type>--`, binds with the routing key, and applies `prefetch_count`. The consumer can come up before any publisher.
• `rpc_service` startup spins up one consumer per subscription. Each delivery is decoded as JSON, wrapped in an `RpcContext` (with the inbound headers stamped on the `RpcCaller`), and handed to the handler. Acks always fire, even if the handler errors — events are best-effort.
• The shared `Semaphore` is now `Arc` so RPC and event consumers share the same `max_workers` budget.

Examples

• `cli_sender` — generic CLI RPC sender:
  `cargo run --example cli_sender -- [arg ...]`
  Args are parsed as JSON when possible, falling back to plain strings, so numbers / booleans / objects can be passed directly.
• `event_observer` — subscribes to `users.user_created` and prints each event as it arrives.

Relationship to other PRs

• Independent of feat: Wire event dispatcher for ctx.events.dispatch #157 (event dispatcher). Code-wise the consumer doesn't import the dispatcher, but to test events end-to-end you'll want both feat: Wire event dispatcher for ctx.events.dispatch #157 and this PR merged. Either order works.

Local test plan

1. `cargo run --example simple_macro`
2. `cargo run --example proxy_service`
3. `cargo run --example cli_sender -- proxy hello Girolle` → expect `"Hello, Girolle!"`.
4. (After feat: Wire event dispatcher for ctx.events.dispatch #157) `cargo run --example event_emitter` and `cargo run --example event_observer` in two terminals.
5. `cargo run --example cli_sender -- users create_user Girolle`
6. The observer prints `[users.user_created] {"name":"Girolle"}`.

Checks

• `cargo build --workspace --examples`
• `cargo test --workspace --lib` (14 passed)
• `cargo test --workspace --doc` (31 + 1 passed — one new doctest for `subscribe`)
• `cargo clippy --workspace --all-targets`

Known limitations (deferred)

• No `#[girolle_event]` macro yet; subscribers construct an `Arc` closure manually. Same shape as the hand-rolled `RpcTask::new` form, so consistent with the rest of the API.
• One handler per `(source, event_type)` per service. Multiple handlers would need queue-name disambiguation; not hard, can follow up if needed.
• Event handler errors are logged but don't nack — best-effort delivery.

🤖 Generated with Claude Code

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