Timers - Implement a Cancelable Delayed Execution Queue

Question

## Problem

Implement a `TimerQueue` class that schedules callbacks to run after a specified delay. It must support cancellation.

```python
import time
import threading

class TimerQueue:
    def schedule(self, delay_ms: int, callback: callable) -> int:
        # Returns a timer_id
        ...

def cancel(self, timer_id: int) -> bool:
        # Returns True if successfully canceled, False if already fired
        ...

def shutdown(self):
        # Cancel all pending timers and release resources
        ...
```

**Expected behavior:**
```
tq = TimerQueue()
id1 = tq.schedule(500, lambda: print("A"))   # fires after 500ms
id2 = tq.schedule(200, lambda: print("B"))   # fires after 200ms
tq.cancel(id1)
# After 200ms: prints "B"
# "A" never fires
```

## Follow-ups

1. How do you implement this using a min-heap to efficiently find the next timer to fire?
2. What is the risk of a race condition between `schedule` and `cancel` and how do you prevent it?
3. How would you implement `reschedule(timer_id, new_delay_ms)`?
4. How does this compare to the JavaScript event loop's `setTimeout`/`clearTimeout` internals?

Timers - Implement a Cancelable Delayed Execution Queue

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