fix(chat): keep the latest reply mounted through settlement and animate the timeline collapse

The streaming and settled versions of the trailing assistant reply were
two components, so settling unmounted one and mounted the other and the
entrance tween replayed over text the reader was already on. One
LatestAssistantEntry now owns that row under a single key, and the
history list excludes it. The CSS entrance keyframe that doubled the
GSAP tween is gone and the tween matches the documented 160ms. The step
timeline no longer remounts on settle: it is a button-controlled
disclosure with a 180ms grid-rows transition, the reader's own toggle
wins over the live default, and an in-flight request with no events yet
shows a queued row instead of an empty shell.

BUG-474 BUG-475

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JUei7K13cYxLHE3Axe4A45

frontend/tests/home-streaming-render-split.test.ts

@@ -4,15 +4,15 @@ import { renderToString } from "react-dom/server";
 import test from "node:test";
 
 import {
+  LatestAssistantEntry,
   SettledMessageList,
-  StreamingMessageEntry,
   UnsplitChatTranscript,
   type ChatTranscriptActions,
   type ChatTranscriptProps,
 } from "../src/components/chat-transcript.tsx";
 import type { ChatMessage } from "../src/lib/chat-message-view.ts";
 import { createStreamFrameBuffer, type StreamFrameScheduler } from "../src/lib/stream-frame-buffer.ts";
-import { streamingChatMessageView } from "../src/lib/chat-message-view.ts";
+import { settledChatMessageViews, streamingChatMessageView } from "../src/lib/chat-message-view.ts";
 import {
   disableHomeStreamingRenderProbe,
   enableHomeStreamingRenderProbe,
@@ -58,7 +58,14 @@ test("the split architecture renders settled history once while streaming tokens
   for (const streamingText of tokens) {
     const streamingMessage = streamingChatMessageView(messages, true, streamingText);
     assert.ok(streamingMessage);
-    renderToString(createElement(StreamingMessageEntry, { message: streamingMessage }));
+    renderToString(createElement(LatestAssistantEntry, {
+      ...base,
+      loading: true,
+      message: streamingMessage,
+      views: [...settledChatMessageViews(messages), streamingMessage],
+      index: messages.length,
+      cancellationPending: false,
+    }));
   }
   const split = homeStreamingRenderProbeSnapshot();
   disableHomeStreamingRenderProbe();
@@ -103,7 +110,13 @@ test("frame coalescing renders the streaming row once per frame, not once per to
     flush: (frame) => {
       const streamingMessage = streamingChatMessageView(messages, true, frame.answer);
       assert.ok(streamingMessage);
-      renderToString(createElement(StreamingMessageEntry, { message: streamingMessage }));
+      renderToString(createElement(LatestAssistantEntry, {
+        ...propsFor(messages),
+        loading: true,
+        message: streamingMessage,
+        views: [...settledChatMessageViews(messages), streamingMessage],
+        index: messages.length,
+      }));
     },
   });
   // 200 one-character tokens arrive four per frame across fifty frames.