docs: rewrite PORT-PLAN for the native SDL3 architecture

Godot-era batch records move verbatim to docs/archive/PORT-HISTORY.md.
README and THIRD-PARTY no longer mention submodules.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -4,10 +4,9 @@
 
 ## macOS
 
-需要 CMake、SDL3、Vulkan loader、MoltenVK、glslc，以及仓库中的第三方子模块：
+需要 CMake、SDL3、Vulkan loader、MoltenVK、glslc（第三方库均已随仓库提供源码，无需子模块）：
 
 ```sh
-git submodule update --init --recursive
 cmake -S . -B build-release -DCMAKE_BUILD_TYPE=Release \
   -DMT_BUILD_NATIVE_RENDER=ON -DCMAKE_PREFIX_PATH=/opt/homebrew
 cmake --build build-release --target mt_native_render -j8