Build a Concurrent Link Checker

• ✓Explain what the event loop does, and why async speeds up waiting but not computing
• ✓Bound concurrency with a semaphore, and find the right limit by measurement rather than by guessing
• ✓Put a deadline on every network call, and cancel work cleanly without swallowing CancelledError
• ✓Retry only failures that can change, with exponential backoff, jitter and Retry-After
• ✓Build a queue-based crawler, and recognise the deadlock a bounded frontier causes
• ✓Add per-host limits, a rate limit and robots.txt handling, and report an unreachable site as unreachable
• ✓Handle errors with TaskGroup and ExceptionGroup, leaving no orphan tasks and keeping partial results on Ctrl-C
• ✓Recognise a blocked event loop, move CPU work off it, and say when async is the wrong tool
• ✓Test async code deterministically with a fake clock and a mock network, and against a seeded ground truth

Build a concurrent network tool that is fast, polite and survives hung hosts and rate limits — and know, with numbers, when async is the right answer and when it is not.