joinCommutesAbsorbsAndIsIdempotent

The rest of the join-semilattice contract — commutativity, idempotence and least-upper-bound — over the causal pool, seeds 0..15, plus the byte law on the commutativity pair.

Breadth, not depth: they read 0 violations on a lattice broken in the way #2086 was broken, where associativity over the same pool reads 500. See LatticeLawHarness.runOtherJoinLaws for what that means, and for why commutativity is asserted with no per-binding waiver.

Separate from the bracketing tests because it is O(pool²) where those are O(pool³): it costs a rounding error beside them, and a red here means something different enough to be worth its own name in the report.