Keypad Codenames

An old door-entry system displays a numeric code, but the staff memorize codes as words using the classic phone keypad mapping (2 → abc, 3 → def, 4 → ghi, 5 → jkl, 6 → mno, 7 → pqrs, 8 → tuv, 9 → wxyz).

Given a string digits containing only characters '2'–'9', return every word the code could spell — one letter chosen per digit, in order. Return them in any order. If digits is empty, return an empty list.