Retry and idempotency policy (NR5)

delivery_id: UUIDv7; dedupe_key: hash(event_id + rule_id + channel_id).
Backoff: exponential with jitter; base 2s, factor 2, max 5 attempts, cap 5 minutes between attempts.
Connectors must be idempotent; retries reuse the same dedupe_key and must not duplicate sends.
Out-of-order acks ignored: only monotonic attempt accepted.
Record retry outcomes in receipts and include attempt count + reason.