ui: follow the bottom, otherwise leave the view alone
The screen scrolled itself with a controller of observers, intervals,
direction heuristics and landing timers, and still managed to move the view
while the reader was reading: any mutation that grew the branch was treated
as a reason to keep following.
The page is the scroller, so it follows when the reader sits at the bottom,
and stays where it is otherwise. A resize observer plus native scroll
anchoring does the rest, and `pointerdown` buys the reader a moment of grace
to be caught mid-drag.
