Count recent hires by department

An HR roster stores one employee per row as "name,dept,year" where year is the four-digit hire year. For a headcount report, count how many employees are in department dept AND were hired in year since_year or later. Example: rows ["ava,eng,2023", "raj,sales,2024", "mia,eng,2025"] with dept "eng" and since_year 2024 give 1.