perf(ui): start streams at newest

Reverse the message stream scroll layout so the viewport naturally starts at the newest messages and keeps older content virtualized. Use sentinel-based edge chasing to make jump-to-top/bottom land reliably despite VirtualItem mounts.

packages/ui/src/styles/messaging/message-section.css

@@ -215,6 +215,18 @@
   align-items: flex-end;
 }
 
+.message-stream-overlay {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  min-height: 0;
+  pointer-events: none;
+}
+
+.message-stream-overlay > * {
+  pointer-events: auto;
+}
+
 .message-scroll-button {
   @apply inline-flex items-center justify-center;
   width: 2.75rem;