perf(ui): enable session timeline virtualization and defer heavy tool content

Virtualize message timeline at 5+ turns. Stabilize virtualizer keys,
add prepend-aware scroll compensation for "Load older", and defer
expanded tool bodies by one animation frame. Tighten initial turn
window to 7 turns for faster session open.

packages/ui/src/components/chat/lib/turns/constants.ts

@@ -3,8 +3,8 @@ export const ACTIVITY_STANDALONE_TOOL_NAMES = new Set<string>(['task']);
 export const HIDDEN_INTERNAL_TOOL_NAMES = new Set<string>(['todowrite', 'todoread']);
 
 export const TURN_WINDOW_DEFAULTS = {
-    initialTurns: 10,
-    batchTurns: 8,
+    initialTurns: 7,
+    batchTurns: 7,
     prefetchBuffer: 16,
 } as const;