updateRelationsBatch previously pre-sorted ops remove->modify->add, which
changed the final state whenever one tuple was touched by mixed kinds:
[add, remove] left the tuple present, [modify, add, modify] ended with
the middle value. Now ops apply strictly in the given order via the
dedup-aware _addRelationInternal (in-place last-write-wins) with upfront
validation, post-batch PLTC edge updates, and per-relation arbiter-level
cache invalidation.
batch-order-parity.test.js pins the contract with an in-order mirror
(modify-of-missing is a silent no-op).
John Dvorak