[round.style] Unclear whether the specification is complete and unambiguous

[tkoeppe]

From the application of Motion 2026-03 LWG 1, issue LWG4474:

If there can be more than 1 "equally near" values with "even least significant digit"s, then we can't chose "the one with an even least significant digit".

Originally posted by @burblebee in #8926

[tkoeppe]

%FIXME: The wording "equally near" seems problematic.
%FIXME: Can multiple values have an "even least significant digit"? Or none at all?