// Package outpost implements the transactional outbox pattern.
//
// The model has three seams:
//
//   - Appender persists messages as part of a unit of work owned by a backend
//     adapter -- a SQL transaction, a CQL batch, a Kafka transaction, or
//     nothing at all. Emit encodes a value as JSON and appends it through one.
//   - Store is the relay's view of a backend: leased delivery, completion,
//     failure, and abandonment of claimed messages.
//   - Producer is what the relay does with a claimed message: send the email,
//     publish to a broker, invoke a handler. ProducerFunc adapts a function.
//
// Handlers are registered on a Dispatcher by message name and can be fed by
// any producer: a relay using ProducerFunc(d.Dispatch), a broker consumer, or
// a poller process. A handler receives the decoded payload and can read the
// message metadata -- its ID, grouping key, headers, and delivery attempt --
// with InfoFromContext.
//
// Guarantees: at-least-once delivery; retries with backoff, spread so that
// independent workers do not retry in lockstep; permanent failures and
// exhausted attempts are abandoned rather than retried; the lease duration is
// derived from batch size, worker count, and delivery timeout so a claimed
// batch cannot outlive its lease.
//
// Non-goals: exactly-once delivery; ordering across messages (a single relay
// with Workers=1 processes claims in store order, and nothing more); codecs
// other than JSON; deferred delivery acknowledgements.
package outpost