perf(ui): widen markdown session cache

Retain detached Markdown DOM for eight recent sessions so medium-frequency session switching can restore more first-screen content without reparsing it.

Limit each session to four cached parts, reducing the worst-case retained fragment count from 36 to 32 while preventing long sessions from dominating the cache.

packages/ui/src/components/chat/markdown/detachedMarkdownDomCache.ts

@@ -27,10 +27,10 @@ type DetachedMarkdownDomCacheLimits = {
 type SessionCache = Map<string, DetachedMarkdownDom>;
 
 const DEFAULT_LIMITS: DetachedMarkdownDomCacheLimits = {
-  // Three buckets cover the common A/B/C recent-session rotation without
+  // Eight buckets cover a broader recent-session working set without
   // coupling eviction to React commit or microtask timing.
-  maxSessions: 3,
-  maxEntriesPerSession: 12,
+  maxSessions: 8,
+  maxEntriesPerSession: 4,
 };
 
 export class DetachedMarkdownDomCache {