Inspector Timeline: make loaded history reversible

[Phoenix500526]

While working on #3986, I found that reversing the Timeline order is only part of the interaction problem. Showing the newest turn first makes recent activity easier to find. However, after a user loads earlier records, those records remain expanded and there is no way to return to the initial view that contains only the recent page. In a long session, a user may load several earlier pages to inspect one older turn. Once their focus is deep in the history, returning to the newest turn requires scrolling through all of the loaded records again.

One possible direction is to make the Timeline header sticky within the Inspector scroll container. The Timeline heading would remain visible while the user scrolls through history. After earlier records have been loaded, the header could expose an action to collapse the loaded history and return to the initial recent-only view. Load earlier records would remain at the end of the Timeline, so users could still load multiple pages when needed. I would like feedback on this interaction model. Is a sticky Timeline header a good way to keep the collapse action reachable? Is there a better way to return from deep history to recent activity?

Here is another related issue #1625. It seems that @ARE404 is working on some relevant topic. Any thouhgt of this?