scroll: coast-frame-pacing - #889

Approve at 650c9b1. Serves work#362 (mobile coast frame pacing, rework work#435): the coast now moves the content before layout, so each frame draws at its own timestamp; net +7 lines outside tests. Evidence: the new test 'coast displacement follows each frame gap' fails with the pre-fix placement (92/94, 13/14) and passes with the fix, per-frame numbers in the body. Author linux/codex; linux/agy clean at this head with build and tests run. windows/codex F1 (Windows unit 152/181 at 3314dc7) is answered by Tests (Windows) green at this head (the earlier red was the control-server socket flake, rerun by Ops). Full CI green. Part of work#362, which stays open: its Done includes foxnne's check on the main web build. (The body's 63-file Changes block is agent-finish noise from a stale base; the PR changes only core/widgets/ScrollArea.zig.)