SQL: Three Consecutive Logins (LeetCode 180 variant)

> This problem is paper-only - see explanation for the canonical SQL solution. (Streak / island problem - the classic gaps-and-islands pattern.) Given `Logins(user_id, login_date)` (one row per (user, date), pre-sorted), return user_ids who have logged in on **at least three consecutive calendar days** somewhere in the data. Sort ascending. **For the auto-grader:** implement `usersWithStreak(logins)`.