S14 Feed Timeline screen. Renders the Timeline produced by
crate::feed_timeline::walk as a scrollable table with cursor
plus selection-detail line. Loading state is driven by
:feed-timeline <owner> <topic> (which spawns the walk and pushes
the result through an mpsc channel into App’s tick handler —
same shape as the durability_tx → S13 Watchlist plumbing).
Shared scroll helpers for components with selectable / overflowing
lists (S2 stamps, S6 peers, S9 tags). Components track a usize
offset alongside their selected index; this module bundles the
“keep selected visible” math + the right-edge scrollbar render.