v0.6.0 — rate-limit handling
Opt-in-by-default 429 retry (RetryPolicy + execute() seam), per-call retry= override, control-plane convergence. Amends ADR-0002 (a 429 on an idempotent call is now retried by default; opt out with SAIAClient(retry=False)). Full notes: docs/CHANGELOG.md.