Function parse_period 

pub fn parse_period(period: &str) -> Result<(String, String), YukiError>

Parse a period string into (start_date, end_date) as “YYYY-MM-DD” strings.

Supported formats:

• "YYYY" — full calendar year
• "YYYY-QN" — calendar quarter (Q1–Q4)
• "YYYY-MM" — calendar month