feat(rounding)!: round half to even instead of half up

Long runs of half-up rounding drifted the reconciled balance upward by
about a cent every four thousand postings.

BREAKING CHANGE: Round() now returns the nearest even multiple on a tie.
Callers that depended on half-up must pass RoundHalfUp explicitly.

Co-authored-by: Miguel Otero <miguel.otero@example.invalid>