Add a shouldStopCallback hook for graceful external stops

[kissifrot]

Add a shouldStopCallback hook for graceful external stops

• Some applications need to stop an active crawl based on external state, such as a database flag, an admin action, or a feature toggle.
• Today, the only graceful stop mechanism is the internal shouldStop flag, which makes these integrations awkward and pushes tests toward mutating internal state.
• This adds a fluent shouldStopCallback() hook that is evaluated before scheduling each next request. When it returns true, the crawl stops gracefully and start() returns FinishReason::Interrupted.

[kissifrot]

Little ping @freekmurze 🙏

[freekmurze]

Clean implementation that follows the existing fluent API patterns. Good phpdoc, the callback is only invoked when set (no perf impact for existing users), and test coverage covers both immediate and mid-crawl stops. CI is green across all matrix jobs.