WordCountService: async startup, resilient error handling, SRP #1609

Startup parses ~400 HTML files synchronously (BuildAsync returns Task.CompletedTask), blocking host readiness. IHostedLifecycleService has five empty methods, and only some exception types are caught per file, so others abort host start. BuildAsync is public on the concrete type only for tests. Consider a BackgroundService/Lazy<Snapshot> and degrade-don't-crash behavior.

Found during review of #1267; intentionally kept out of that PR.