refactor(home): 校正面 15 个状态从 Home 下沉

Home() useState 66->52, useRef 41->39. Shell-facing fields merge into one object;
subtree state lives in useRectificationSurface. Chat props 20->7. Growth contract
rebaselined. Zero behavior/copy change.

frontend/tests/rectification-surface-contract.test.ts

@@ -35,7 +35,10 @@ test("the rectification surface is revealed once: Case hydration precedes the sw
   // Panel: one key per binding, snapshot and turns as initial state, later turns as a prop update.
   assert.match(page, /key=\{`\$\{rectificationSessionId\}-\$\{rectificationCaseId\}`\}/);
   assert.doesNotMatch(page, /"ready" : "loading"/);
-  assert.match(page, /initialSnapshot=\{rectificationSnapshot\}/);
+  // 原值：initialSnapshot={rectificationSnapshot} 写在 page.tsx JSX
+  // 新值：initialSnapshot: rectificationSnapshot 写在 hook 的 panel
+  // 原因：snapshot 只服务子树，随 panel 下沉
+  assert.match(page, /initialSnapshot: rectificationSnapshot/);
   assert.match(page, /declaredTime=\{rectificationDeclaredTime\}/);
   assert.match(wrapper, /initialSnapshot: RectificationCaseSnapshotPayload \| null;/);
   assert.match(chat, /useState\(initialSnapshot !== null\)/);