Treat vertical tab and form feed as whitespace in the KSS fuzz check

The lexer skips them, so the formatter may drop them, but the fuzz check
counted that as a change beyond whitespace (Kryon CI, seed 1113). The
check now uses the lexer's KssIsSpace, and the sheet joins the corpus.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>