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/lib/persistence.ts

@@ -531,6 +531,7 @@ const materializeAuthoritativeUiSettings = (settings: DesktopSettings): DesktopS
     darkThemeId: DEFAULT_DARK_THEME_ID,
     openInAppId: DEFAULT_OPEN_IN_APP_ID,
     showReasoningTraces: defaults.showReasoningTraces,
+    streamingAutoFollowEnabled: defaults.streamingAutoFollowEnabled,
     workStatusPanelEnabled: defaults.workStatusPanelEnabled,
     workStatusHiddenSections: defaults.workStatusHiddenSections,
     sessionRecapEnabled: defaults.sessionRecapEnabled,
@@ -637,6 +638,9 @@ const applyDesktopUiPreferences = (settings: DesktopSettings) => {
   if (typeof settings.showReasoningTraces === 'boolean' && settings.showReasoningTraces !== store.showReasoningTraces) {
     store.setShowReasoningTraces(settings.showReasoningTraces);
   }
+  if (typeof settings.streamingAutoFollowEnabled === 'boolean' && settings.streamingAutoFollowEnabled !== store.streamingAutoFollowEnabled) {
+    store.setStreamingAutoFollowEnabled(settings.streamingAutoFollowEnabled);
+  }
   if (typeof settings.sessionRecapEnabled === 'boolean' && settings.sessionRecapEnabled !== store.sessionRecapEnabled) {
     store.setSessionRecapEnabled(settings.sessionRecapEnabled);
   }
@@ -1158,6 +1162,9 @@ const sanitizeWebSettings = (payload: unknown): DesktopSettings | null => {
   if (typeof candidate.showReasoningTraces === 'boolean') {
     result.showReasoningTraces = candidate.showReasoningTraces;
   }
+  if (typeof candidate.streamingAutoFollowEnabled === 'boolean') {
+    result.streamingAutoFollowEnabled = candidate.streamingAutoFollowEnabled;
+  }
   if (typeof candidate.sessionRecapEnabled === 'boolean') {
     result.sessionRecapEnabled = candidate.sessionRecapEnabled;
   }