Merge main (anchored-turn chat scrolling) into perf/switch-and-scroll

Main replaced the chat timeline scroll engine while this branch was in
flight, which obsoletes two of its subareas and reshapes a third:

- Chat timeline: main's LegendList-based MessageList/ChatContainer win;
  the activation-overscan staircase targeted the removed tanstack path
  (LegendList provides adaptive rendering natively) and is dropped along
  with its test.
- Scroll shadows: main's hook-based masks stay (the virtualized list owns
  its scroll element — there is no wrapper to hand the styling to); the
  viewport-wrapper ScrollShadow rewrite, its index.css replacement, its
  test, and the call-site viewportClassName adaptations are reverted to
  main. The chat OverlayScrollbar keeps this branch's disableHorizontal.
- OverlayScrollbar: the direct-DOM rewrite lands, but binding now follows
  the live container node instead of binding once per ref object — the
  chat scroller remounts on every session switch, and a bind-once
  contract left the scrollbar attached to a dead element.
- Markdown renderer: the detached-DOM cache and warm-block fast path
  merge with main's block-commit reveal (enter cascade), streaming code
  highlighting, and gutter reservation; the per-block reconcile keeps
  both the decoration-refresh path and the reveal cascade.

packages/ui/src/components/multirun/MultiRunLauncher.tsx

@@ -447,7 +447,7 @@ export const MultiRunLauncher: React.FC<MultiRunLauncherProps> = ({
         </header>
       ) : null}
 
-      <ScrollShadow viewportClassName="flex-1 min-h-0" className="flex-1 min-h-0 overflow-auto" size={64} hideTopShadow>
+      <ScrollShadow className="flex-1 min-h-0 overflow-auto" size={64} hideTopShadow>
         <div className="mx-auto w-full max-w-2xl px-4 sm:px-6 py-5">
           <div className="flex flex-col gap-5">