Summary

The proposer recently gained two key improvements for startup performance and resilience:

1. Backward sync from latest game to anchor fix: avoid full replay on restarts by backward sync #681
2. State backup/restore for fast recovery feat(fault-proof): proposer state backup for faster recovery #756

The challenger (challenger.rs) would benefit from the same treatment. Currently it re-syncs all games from scratch on every restart, which is slow and wasteful — especially on chains with high dispute game volume.