merge

Merge an ORSet received from another replica (via anti-entropy / gossip) into this exporter's state, then flush the merged result to store.

If a WarpCausalClock was supplied, the remote replica's causal frontier is folded into the local clock first, so the next auto-stamped span records those remote dots as predecessors — the cross-replica happens-before path (#846).

Idempotent: merging the same set twice produces the same result.