Scrolled

Posted whenever the chat's vertical scroll offset changes.

Transcript hydration keys off the actual scroll offset instead of the scrollbar ScrollUp/ScrollDown messages, because those never reach the app for the common scroll paths: wheel/trackpad scrolling arrives as MouseScroll* events, keyboard scrolling runs through key-binding scroll actions, and both move scroll_y directly without posting a scrollbar message. Scrollbar-track clicks do post ScrollUp/ScrollDown, but this container's own _on_scroll_up/_on_scroll_down handlers consume them via event.stop() before they can bubble to the app. Watching scroll_y covers every input device uniformly. Validated against Textual 8.2.7.