Msys2 detect / ofMenu / vs - #8657

Fixes: #8640

MSYS2 terminal:

of.sh: detect MSYS2 shells as msys2, pick the MSYS2 env in the menu

Fixes #8640

Problem

When of.sh runs from an MSYS2 UCRT64 / CLANG64 / CLANGARM64 shell, ofMenu reports vs / 64, and setup downloads the Visual Studio libs instead of the msys2 ones.

Changes (scripts/of.sh only)

Detection

• Git Bash is identified by git-bash.exe / git-cmd.exe at its root, as suggested in the issue. MSYSTEM alone can't separate the two, because Git Bash also sets MSYSTEM=MINGW64.
• An MSYS2 shell is detected by pacman, or by MSYSTEM being ucrt64|clang64|clangarm64|mingw64.
• The plain MSYS shell falls back based on the CPU.

Native ARM64 detection

• On Windows on ARM, both Git Bash and MSYS2's own bash run as emulated x64 and report PROCESSOR_ARCHITECTURE=AMD64 / uname -m x86_64. That's why ARM64 machines showed vs / 64.
• hostIsArm64 now reads the native CPU from the registry first (HKLM\…\Session Manager\Environment\PROCESSOR_ARCHITECTURE, which is not emulated). It then falls back to MSYSTEM, PROCESSOR_ARCHITEW6432 and uname.
• detectHostVsArch and the msys2 environment default both use it. On ARM64 the vs libs default becomes arm64, which makes download_libs pull x64 + arm64 + arm64ec.

Menu: choosing msys2 libraries now asks for the environment

MSYS2 environment?
  ucrt64      — GCC, UCRT (x64)  (detected)
  clangarm64  — Clang, UCRT (ARM64)
  clang64     — Clang, UCRT (x64)

• Inside MSYS2, the detected environment is listed first as the default.
• From Git Bash, the CPU default (clangarm64 / ucrt64) is listed first, marked (default).
• mingw64 is listed only when it's the detected environment.

Downloads

• The chosen or detected environment is passed to download_libs as -a <env>. That way, picking msys2 from Git Bash no longer falls back to Git Bash's MSYSTEM=MINGW64.
• The value is exported only for that one download, so it can't leak into a later vs download in the same menu session.
• A LIB_ARCH the user sets explicitly (LIB_ARCH=clang64 of update libs msys2) is still respected.
• The help text lists LIB_ARCH=ucrt64|clang64|clangarm64|mingw64 for msys2.

MSYS2 without git

• MSYS2 doesn't ship git, so the menu showed a git checkout as nightly package (no .git) and offered clone().
• When git isn't on PATH, of.sh now uses Git for Windows' git.exe (Program Files or the per-user install). MSYS2 converts the /c/… path arguments for it.
• With no git at all, a folder with .git shows git checkout (git not found — pacman -S git). clone() is offered only when there's no .git at all.

Menu

• vsconfig() is shown only for the vs platform, not in MSYS2 shells. of vsconfig still works from any Windows shell.
• OF menu version bumped to 0.4.6.

Windows ARM64

• Host Windows ARM64 even when run in git bash (x64) will correctly identify host platform as arm64.