Not Every Byte Gets a Vote

# Summary When implementing replay functionality in a deterministic game engine, the author discovered that naively hashing all world state to detect divergence causes false negatives when non-gameplay fields (like debug data or render caches) change without affecting actual gameplay. To solve this, they categorized each field by its role—authoritative gameplay state, derived caches, observations/debug output, and presentation state—so the checksum only validates genuinely meaningful state changes. The engine uses fixed-tick simulation with clearly defined phases (ingress, control, derive, plan, apply, cleanup) to create predictable checkpoints for replay validation.