Lightyear Deterministic Replication

Utilities for lockstep-style deterministic simulation on top of Lightyear:

• ChecksumSendPlugin / ChecksumReceivePlugin compute and verify XOR checksums of prediction history across client and server.
• LateJoinCatchUpPlugin lets a client that connects mid-game request a one-time snapshot of a remote entity's state so it can fast-forward to the current tick via a forced rollback.
• DeterministicReplicationPlugin wires up the shared archetype index used by both features.