fix(rectification): stop re-parsing settled messages on every stream frame

frontend/tests/chat-markdown-split.test.ts

@@ -4,7 +4,7 @@ import test from "node:test";
 import { createElement } from "react";
 import { renderToString } from "react-dom/server";
 
-import { StreamingMarkdown } from "../src/components/chat-message-content.tsx";
+import { StableMarkdownPrefix, StreamingMarkdown } from "../src/components/chat-message-content.tsx";
 import { splitStableMarkdown } from "../src/lib/chat-markdown-split.ts";
 
 const contentSource = readFileSync(new URL("../src/components/chat-message-content.tsx", import.meta.url), "utf8");
@@ -68,9 +68,22 @@ test("the streaming renderer parses the prefix and the tail as two separate docu
   assert.match(contentSource, /const StableMarkdownPrefix = memo\(function StableMarkdownPrefix/);
   assert.match(contentSource, /splitStableMarkdown\(text\)/);
   assert.match(contentSource, /streaming\s*\?\s*<StreamingMarkdown/);
-  // 原值: `: renderMarkdown ? renderMarkdown(spoken)` 只给口语层走 markdown。
-  // 新值: 口语层与报告层都走 `renderProse`。
-  // 原因: 两层可见性后，折叠内外共用同一套解析，避免两套 fallback。
-  assert.match(contentSource, /: renderProse\(spoken, renderMarkdown\)/);
+  // 原值: `: renderProse(spoken, renderMarkdown)` 结算口语层每次重渲都全量 parse。
+  // 新值: 结算口语层走 `StableMarkdownPrefix`（按文本记忆 + 解析缓存）。
+  // 原因: BUG-725 流式帧不得重跑已结算 Markdown。
+  assert.match(contentSource, /: <StableMarkdownPrefix text=\{spoken\} renderMarkdown=\{renderMarkdown\} \/>/);
   assert.match(messageRowSource, /streaming=\{message\.state !== "settled"\}/);
 });
+
+test("settled markdown parses the same text only once across consecutive renders", () => {
+  const calls: string[] = [];
+  const renderMarkdown = (text: string) => {
+    calls.push(text);
+    return createElement("p", null, text);
+  };
+  const text = "已结算的一段回答，对照了 D9 和 D10。";
+  renderToString(createElement(StableMarkdownPrefix, { text, renderMarkdown }));
+  renderToString(createElement(StableMarkdownPrefix, { text, renderMarkdown }));
+  assert.equal(calls.length, 1);
+  assert.deepEqual(calls, [text]);
+});