causalFloor

The elementwise max of both components' compaction floors — the floor counterpart of the causalDots union above.

Sound because a dot at-or-below the merged floor is at-or-below A's floor or at-or-below B's, so some component delivered it and therefore the product did — and it is the highest floor the two components' own floors justify. Without it a product wrapping a floored Rga would inherit the empty default while its causalDots no longer carries the floored dots, and a consumer folding a delivered frontier would under-report it. Both components default to VersionVector.EMPTY, so this is a no-op for every non-compacting pair.