df has columns month (strings like "2024-03") and revenue, with possibly missing months. Implement yoy(df) returning a DataFrame with columns month, revenue and yoy_pct: the percentage change versus the same month one year earlier, rounded to 1 decimal place, or NaN if that month isn't in the data. Sort by month.
Match on the actual calendar month, not on row position (missing months would break shift(12)).