ci: use native runner for Windows ARM64 releases

[meop]

Summary

• Run the existing aarch64-pc-windows-msvc release and test jobs on GitHub native windows-11-arm runners.
• Add the Windows ARM64 GNU-family Rust target, aarch64-pc-windows-gnullvm. Rust does not provide a literal aarch64-pc-windows-gnu target.
• Package every Windows target by target triple, so ARM64 binaries are archived as ZIP files with sd.exe.

Toolchain

The GNU-family target installs the MSYS2 CLANGARM64 toolchain and uses the setup action output for Cargo linker configuration. This avoids a runner-specific hard-coded path.

The gnullvm build is linked with -C target-feature=+crt-static. Without it, sd.exe imports MSYS2's libunwind.dll and does not start on a machine without MSYS2. The existing x86_64-pc-windows-gnu build is unaffected; Rust links its unwinder statically.

Validation

• Existing MSVC native-runner smoke: successful build and archive.
• GNU-family native-runner smoke with static libunwind: build, import check, and sd --version with only System32 on PATH.
• Test workflow on this branch: passed.

The fork-only validation workflow is not included in this PR.

Disclosure

This change was prepared with assistance from Claude (Anthropic) and OpenAI Codex. I reviewed the diff and validated it through the linked GitHub Actions runs.

[meop]

Pushed a fix for the GNU-family build: the previous gnullvm artifact imported MSYS2's libunwind.dll, so it would not have started on a clean Windows ARM64 machine. It is now linked with +crt-static, and the fork smoke checks that sd.exe runs with only System32 on PATH. Validation links are updated in the description.