Implement a web crawler that starts from a given URL and visits every reachable page on the same host. You are given: - `start_url`: a string representing the first page to crawl. - `fetch_links(url) -> list[str]`: a blocking function that returns all URLs found on the page. Requirements: 1. Crawl pages concurrently using multiple threads. 2. Only visit URLs whose host is exactly the same as the host of `start_url`. 3. Visit each eligible URL at most once, even if multiple pages link to it. 4. Return all visited URLs in any order. 5. Your implementation must be thread-safe. You may assume: - `fetch_links` is thread-safe. - URL strings are valid. - The crawl graph is finite. Discuss the main synchronization challenges, such as protecting the shared visited set and coordinating worker threads efficiently.