feat(code-runtime-python): make the TypedDict wire mirror an executable gate

Address the two standing review suggestions in this layer rather than deferring
them to PR #4:

- Extend tests/protocol-mirror.e2e.ts to read each py/protocol.py TypedDict's
  required/optional key set and assert it against the wire field names
  src/protocol.ts declares (global included, via functional TypedDict). The
  round-12 class of drift — a renamed/dropped field, or one side making a field
  optional the other requires — now fails a test instead of relying on review.
  Field types remain review-guarded (no mechanical TS/Python equivalent).
- Drop the forward references to PR #4's internal mechanisms from this layer's
  prose: the "256 MiB frame ceiling" figure and the "(index.ts)" fd-3 pinning
  citation become an abstract "host-side inbound frame-size cap" so the JSDoc,
  spec, README, and Agent Note describe only what this layer owns.

Update both README sides and the Agent Note (both languages) to state the
mirror is now executable, and re-record their i18n pairings.

packages/code-runtime/code-runtime-python/README.zh.md

@@ -25,5 +25,5 @@ host 与 CPython 子进程在子进程的 fd 3 上交换一个无版本号的 JS
 
 ## Known Limitations and Deferred Work
 
-- **跨语言 guard 只覆盖两个运行时执行的面** —— `PROTOCOL_FD` 与日志截断标记。`py/protocol.py` 中的 `TypedDict` 帧形状靠 review 而非自动化检查来镜像 `src/protocol.ts`：跨 TypeScript 与 Python 比较类型声明在此无机械等价物，故未来的形状漂移由 review 加后端真子进程套件捕获，而非本包的测试。
+- **跨语言 guard 覆盖运行时执行的面与帧字段形状** —— `tests/protocol-mirror.e2e.ts` 启动一个真实 `python3`，对照 `src/protocol.ts` 断言 `PROTOCOL_FD` / 日志截断标记文本，以及 `py/protocol.py` 中每个 `TypedDict` 的必填/可选 wire 字段集。它不比较字段的*类型*（例如 `cpuSeconds` 两侧都是 `int`）：跨 TypeScript 与 Python 比较类型声明在此无机械等价物，故类型级漂移仍由 review 加后端真子进程套件捕获，而非本包的测试。
 - **`PythonCodeRuntime` 实现与 Python 侧 JSON codec 不在本层** —— 它们在基于本分支的 backend-core PR 中交付；在那之前 `src/index.ts` 只 re-export 协议词汇。