Rust tasks

Derive typed JSON tasks and create durable envelopes.

use headgate::Task;

#[derive(serde::Serialize, serde::Deserialize, Task)]
#[task(kind = "invoice.generate", version = 1)]
struct GenerateInvoice { invoice_id: String }

let task = GenerateInvoice { invoice_id: "inv-42".into() };
let payload = task.encode()?;
let envelope = headgate::Envelope {
    id: "invoice-job-42".into(),
    kind: GenerateInvoice::TYPE.into(),
    schema_version: GenerateInvoice::VERSION,
    fingerprint: headgate::fingerprint(GenerateInvoice::TYPE, &payload),
    payload,
    queue: "billing".into(),
    partition_key: "tenant-7".into(),
    ..Default::default()
};

Kind and schema version are durable public contracts. Keep the kind stable and use aliases and upcasters when deployed payloads need to evolve.