NewCursorFromFlags validated --first/--last, --after/--before, and
--after/--last as mutually exclusive, but missing the symmetric
--first/--before guard. Passing --first with --before fell into the
backward branch, where size defaulted from the unset --last and the
direction silently flipped to Tail, discarding --first and reversing
pagination. Reject the combination up front like the other invalid
pairings.
Signed-off-by: Émile Ré <emile@probo.com>
87e95e8993