docs(packaging): 记录 Termux glibc 构建与 termux-x11 运行配方

新增 packaging/docs/termux-glibc-build.md：glibc 工具链（Rust gnu 组件 +
patchelf + clang/lld + Debian libgcc crt）、构建环境（LD_PRELOAD/CA/USTC 镜像）、
termux-x11 运行与截图取证、GPU 现状与已知边界。
支持矩阵四处同步：Termux 行从「GUI 未验证」改为「源码自建、CPU 渲染单机验证过」，
仍标注实验级、不在发布矩阵内。

README.md

@@ -194,8 +194,9 @@ This list is the reason to believe the rest. None of it is hidden behind marketi
   report lists every conflict so nothing is overwritten silently.
 - **Install progress is stage-level**, not byte-level, on the content-library install path.
 - **No native file chooser.** Import takes drag-and-drop or a pasted path.
-- **Android/Termux is experimental and not promised**: no released asset targets it, and the GUI is
-  unverified there.
+- **Android/Termux is experimental and not promised**: no released asset targets it. A GUI build is
+  possible from source via the glibc toolchain (`packaging/docs/termux-glibc-build.md`); it was
+  verified once on CPU rendering only.
 
 ## Download and run