fix: stabilize chat history prepend scroll preservation on mobile and desktop

- Mobile: defeat iOS momentum scroll when compensating history prepend
  (overflow toggle + short rAF watchdog); disable history virtualization
  and post-paint background prepends; preload Markdown renderer and use
  plain-text Suspense fallback to avoid first-frame geometry shifts
- Desktop: stop double-compensating prepends on the virtualized list -
  virtua shift owns the adjustment; remove sticky-anchor heuristics that
  misfired as failed restores
- Sync: skip no-op store writes when messages/parts are unchanged

packages/ui/src/components/chat/markdownRendererLoader.ts

@@ -0,0 +1,13 @@
+let markdownRendererModulePromise: Promise<typeof import('./MarkdownRendererImpl')> | null = null;
+
+export const loadMarkdownRendererModule = () => {
+  markdownRendererModulePromise ??= import('./MarkdownRendererImpl').catch((error) => {
+    markdownRendererModulePromise = null;
+    throw error;
+  });
+  return markdownRendererModulePromise;
+};
+
+export const preloadMarkdownRenderer = () => {
+  void loadMarkdownRendererModule().catch(() => undefined);
+};