Function validate_issue_dates 

pub fn validate_issue_dates(
    start_date: Option<&str>,
    due_date: Option<&str>,
) -> Result<()>

Validate a (start_date, due_date) pair for an issue (FR-007, C-002).

Each present date must parse as a calendar date; when both are present, due_date >= start_date must hold. Returns a config-class error naming both fields on an ordering violation, matching contracts/file-format-planning.md.