Count completed car journeys from sensor logs

You are given a time-ordered list of highway sensor logs. Each log entry contains: - `timestamp` (integer; strictly increasing) - `sensor_type` (one of `"ENTRY"`, `"EXIT"`, `"CHECKPOINT"`) - `car_id` (string) A **journey** is defined as: for the same `car_id`, an `ENTRY` event followed later by an `EXIT` event. `CHECKPOINT` events may occur in between but do not affect the definition. A car may make multiple journeys over time (e.g., `ENTRY ... EXIT ... ENTRY ... EXIT`). If a car has an `ENTRY` without a subsequent `EXIT` by the end of the logs, that partial journey does **not** count. If an `EXIT` occurs when the car is not currently in an active journey (i.e., there was no unmatched prior `ENTRY`), ignore it. Write a function that returns the total number of completed journeys across all cars. Example: Logs: 1. (1, "ENTRY", "A") 2. (2, "CHECKPOINT", "A") 3. (3, "EXIT", "A") 4. (4, "EXIT", "B") 5. (5, "ENTRY", "A") 6. (6, "ENTRY", "B") 7. (7, "EXIT", "B") Result: 2 (A completes 1 journey at t=3; B completes 1 journey at t=7).