pub fn validate_date(s: &str) -> boolExpand description
A strict YYYY-MM-DD calendar-date check (shape + real month/day with
per-month day counts and leap years, year >= 1970), so 2026-13-99,
2026-02-30 and 1969-12-31 are all flagged as malformed. The shared
validation path for date-bearing config and frontmatter fields (FR-007).