ui: reduce per-token render cost when streaming (#26053)

* performance harness - the empirical root

Assisted-by: Claude Opus 4.8

* 210.36ms -> 2.67ms per streamed token

Assisted-by: Claude Opus 4.8

* 11.58ms -> 0.62ms per streamed token

Assisted-by: Claude Opus 4.8

* 22.02ms -> 3.33ms per streamed token

Assisted-by: Claude Opus 4.8

* 3.07ms -> 1.36ms per streamed token at 40 messages

Assisted-by: Claude Opus 4.8

---------

Co-authored-by: Zach Winter <dmtommy@icloud.com>

tools/ui/tests/client/components/ChatMessagesPerfWrapper.svelte

@@ -0,0 +1,12 @@
+<script lang="ts">
+	// Mounts the real ChatMessages list against the real conversations store, so
+	// the harness exercises `displayMessages` (which rebuilds every message's
+	// toolMessages array) rather than a single message subtree.
+	import * as Tooltip from '$lib/components/ui/tooltip';
+	import ChatMessages from '$lib/components/app/chat/ChatMessages/ChatMessages.svelte';
+	import { conversationsStore } from '$lib/stores/conversations.svelte';
+</script>
+
+<Tooltip.Provider>
+	<ChatMessages messages={conversationsStore.activeMessages} />
+</Tooltip.Provider>