set

Write value for key tagged with (timestamp, replica).

Precondition — tag uniqueness. The (replica, timestamp) pair MUST uniquely identify this write for the given key. Reusing the same (replica, timestamp) across two writes to the same key with different values produces non-deterministic convergence under merge — which value survives depends on merge order, not write order. Use a monotonic timestamp source per replica and never reuse a (replica, timestamp) pair. Not enforced at runtime.