Fix ReDoS hang in dimension shorthand expansion

[grosser]

RE_FUNCTIONS (used by expand_dimensions_shorthand!) backtracked exponentially on unclosed CSS functions: ~30 bytes like margin: x() calc(aaaa... pinned a CPU for hours. (The x() prefix gets the value past the unmatched_open_parenthesis? check in parse_declarations!.)

• regex: recursion-first alternation + possessive quantifiers, so failed matches cost O(n) per attempt instead of O(2^n); a function name before ( is never swallowed by a possessive text run, keeping nested calc(var(...)) matching intact
• 10ms regexp timeout as backstop: on timeout, fall back to plain whitespace splitting so hostile values mis-render instead of hanging the process (budget covers a whole scan, so residual quadratic cost is capped too)
• no output change on benign values (verified against the old regex on nested calc/var/url cases); regexp speed within ~10-25% of the old one

Pre-Merge Checklist

• CHANGELOG.md updated with short summary