kafrust v0.2.6

Summary\n\n- Added the opt-in bounded consumer-group commit worker with interval flush, per-partition coalescing, coordinator retry handling, rejoin synchronization, and orderly shutdown.\n- Published matching kafrust-protocol and kafrust crates at 0.2.6.\n\n## Breaking changes\n\nNone.\n\n## Migration notes\n\nUse ConsumerGroup::spawn_commit_worker(Duration) for interval-based queued commits. Existing explicit commit APIs remain available.\n\n## Compatibility evidence\n\n- CI on stable and Rust 1.81: https://github.com/TaeeunKil/kafrust/actions/runs/31564724174\n- Live Kafka Smoke across Kafka 3.7.2, 3.8.1, 3.9.1, 4.3.1, TLS, SASL, ACL, and multi-broker failover: https://github.com/TaeeunKil/kafrust/actions/runs/31563953123\n- crates.io fresh-project default and TLS feature checks completed after publication.\n\n## Known limits\n\n- The API remains pre-1.0.\n- Production OAuth/OIDC provider qualification, broader transaction/KIP-848 failure matrices, and partition queue splitting remain open.\n