Dragging the Git Log scrollbar through a long history stuttered and showed
blank rows: the native scrollbar scrolls asynchronously in WebKit, so the
virtual list rendered rows a frame or more after the new offset was painted.
The list now uses a scrollbar drawn in the DOM (`Scrollbar` in `src/ui`) that
sets the offset from pointer events and notifies scroll listeners at once, so
the virtualizer renders the matching rows before the next paint. Rows are
keyed by slot so a large jump updates the mounted rows instead of rebuilding
them, overscan is lowered, and rows rendered mid-scroll draw their graph
without edge hit targets, which come back once scrolling stops.
Adds a `gitLogScrollbar` perf scenario with budgets.
