idemstep v0.11.0

Stripe's Idempotency-Key, but for your browser agent: wrap a transactional browser step with a client-generated key so a self-healing retry after a slow-but-successful submit is deduplicated instead of placing the order twice. v0.11.0 adds a --version CLI flag, a single-source-of-truth VERSION constant + lockstep test, and backfills the CHANGELOG.