Come up with a better explanation for round(-half-up) vs round-half-down in decompose

[jammychiou1]

Decompose() naturally needs round-half-down. It can be computed with rounding mulhi which always round half up, but our current explanation (here for example) doesn't justify this very clearly. See https://github.com/pq-code-package/mldsa-native/pull/411#discussion_r2371057773 and https://github.com/pq-code-package/mldsa-native/pull/629#discussion_r2508911467 for more context.