refactor(rectification): split the chat component into hooks, parameter functions and blocks

Move-only (TASK-rectification-code-split-20260926 T1/T4). No behavior, API,
copy, DB, Skill or scoring change.

- rectification-agentic-chat.tsx: 2043 -> 737 lines; component body
  1625 -> 630; useState 35 -> 12, useRef 16 -> 9, effects 9 -> 5,
  useCallback 12 -> 6.
- Snapshot sync -> hooks/use-rectification-case-snapshot.ts; board layout
  and live label clock -> two small hooks.
- send / submitStructuredChoice / acceptCandidate bodies ->
  lib/rectification-chat-{turn,choice,accept}-run.ts parameter functions
  (bodies verbatim, deps destructured to the same names); copy/regenerate,
  question repair, pure transcript and snapshot helpers and the per-render
  view derivation -> lib/rectification-chat-*.ts (no React hooks).
- Question-gap blocks and the read-only range line ->
  components/rectification-question-gap-notices.tsx.
- Dependency arrays kept exactly as before (lint warnings +3, listed in
  PROGRESS); no dep added to appease the linter.
- Tests: whole-source contracts read tests/rectification-chat-surface.ts
  (container + split files, like home-surface.ts); slices of moved code now
  call the extracted functions or render the extracted block, each with a
  原值/新值/原因 comment. New tests/rectification-growth-contract.test.ts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017eEAG8HD3mm8gsKXgk8uU8

frontend/tests/rectification-settled-render-split.test.ts

@@ -18,11 +18,10 @@ import {
   homeStreamingRenderProbeSnapshot,
   resetHomeStreamingRenderProbe,
 } from "../src/lib/home-streaming-render-probe.ts";
+import { rectificationChatSurface } from "./rectification-chat-surface.ts";
 
-const chatSource = readFileSync(
-  new URL("../src/components/rectification-agentic-chat.tsx", import.meta.url),
-  "utf8",
-);
+// 原值: 读 rectification-agentic-chat.tsx 单文件。新值: rectificationChatSurface（容器 + 拆出的 hook / 参数函数 / 块，见 rectification-chat-surface.ts）。原因: TASK-rectification-code-split-20260926 只搬不改，整文件断言跟着代码走；切片断言另行改为调用函数。
+const chatSource = rectificationChatSurface;
 const entrySource = readFileSync(
   new URL("../src/components/rectification-message-entry.tsx", import.meta.url),
   "utf8",