feat(termux): 横屏自适应窗口、安装后清理 apt 工作区、补分支 README

- app/boot/termux_window：Termux 上默认窗口按 X 屏幕等比自适应
  （横屏铺满、竖屏不溢出，最大 2×；日志 source=termux-screen），
  ULL_WINDOW_SIZE 覆盖与 1180×760 验收基准不变；5 条单测
- termux::install 成功后清理 $PREFIX/tmp/ull-java-termux（一次安装约 100 MB）
- 构建文档补 clippy/rustfmt 组件、cargo-clippy 目标被带成 android 的坑、
  门禁命令、横屏行为与 openjdk-17 实测
- 新增 README.termux.md（分支定位/已验证矩阵/快速开始/边界/路线图），
  两份主 README 的 Termux 条目链过去

README.md

@@ -195,8 +195,8 @@ This list is the reason to believe the rest. None of it is hidden behind marketi
 - **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. 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.
+  possible from source via the glibc toolchain (`packaging/docs/termux-glibc-build.md`, branch notes:
+  [`README.termux.md`](README.termux.md)); it was verified once on CPU rendering only.
 
 ## Download and run