docs: 目标平台收窄为 macOS arm64 + Android arm64

用户决定：Linux 和 Windows 不做，不验收、不作为任何步骤的阻塞项。

- PORT-PLAN 第 1 节写明范围；2A step 3 的平台列表、2P step 1/2 措辞跟着改；
  2P step 4 由「五个平台分别验证」改为「两个目标平台」，Linux/Windows 两行
  划掉但保留原因备查（上游 2.7 只支持 MSVC + PC/pyconfig.h，mingw 下
  posixmodule.c / dynload_win.c 与 MinGW 头文件冲突）。
- PLATFORMS.md 新增 Scope 段；THIRD-PARTY.md 的 CPython Windows 段由
  "not built yet" 改为 out of scope；PYTHON-EMBED-EVAL.md 下一步两条。
- 仓库里的 mingw-w64 交叉编译只留作可移植性门禁（它抓到过
  UserInterface/StdAfx.h 的真问题），不代表支持 Windows。
- iOS arm64 暂按次要目标记录：保持交叉编译干净，但不作为验收目标。

仅文档改动。port_map.py check 0 error；参考 EterPack.cpp 的 8 个 pack key
值逐个 git grep --cached -w，均未入库。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/THIRD-PARTY.md

@@ -132,11 +132,13 @@ one via a 1-line shim header, see below).
   (2.7's configure refuses a darwin cross build) and drops process control plus
   `<sys/random.h>`/`getentropy`, but keeps `HAVE_GETGROUPS` because
   `posixmodule.c` defines `MAX_GROUPS` — which its `setgroups` path needs — only
-  inside that `#ifdef`. **Windows is not built yet**: upstream supports MSVC +
-  `PC/pyconfig.h`, while our Windows gate cross-compiles with MinGW, where
-  `PC/pyconfig.h`'s gnu-win32 branch leaves `HAVE_UNISTD_H` unset and
-  `posixmodule.c` / `dynload_win.c` then collide with the MinGW headers. Decided
-  in 批次 2P step 4 (PORT-PLAN).
+  inside that `#ifdef`. **Windows is out of scope** (decided 2026-09-23, PORT-PLAN §1: the
+  targets are macOS arm64 and Android arm64). It is also not buildable as-is:
+  upstream supports MSVC + `PC/pyconfig.h`, while the mingw-w64 portability gate
+  cross-compiles with MinGW, where `PC/pyconfig.h`'s gnu-win32 branch leaves
+  `HAVE_UNISTD_H` unset and `posixmodule.c` / `dynload_win.c` then collide with
+  the MinGW headers. So that gate simply builds no `mtpython`, and `port_logic` /
+  `port_platform` drop ScriptLib there.
 * **License:** PSF License Agreement (`cpython-2.7.18/LICENSE`). Ship-safe; the
   only obligation is keeping the license and a change notice, which this section
   and the patched files' comments serve.