Rust worker

Register handlers and run a capacity-aware worker with graceful shutdown.

let mut registry = headgate::Registry::new();
registry.register::<GenerateInvoice, _, _>(|ctx, task| async move {
    ctx.log(format!("generating {}", task.invoice_id));
    Ok(())
})?;

let (worker, handle) = headgate::Worker::new(
    store,
    registry,
    headgate::WorkerConfig {
        queues: vec!["billing".into()],
        capacity: 16,
        ..Default::default()
    },
);

worker.run().await?;

Use the returned handle from normal process signal handling. Shutdown stops new admissions, continues lease renewal during the bounded drain, and releases unfinished jobs cleanly.