feat: add Windows support to zemo upgrade

[hidetzu]

Closes #11.

Summary

• assetName() now returns zemo-x86_64-windows.zip on Windows x86_64.
• New extractZemoBinaryFromZip helper: stages the downloaded .zip to a sibling temp file (because std.zip.Iterator requires a *File.Reader), iterates the central directory, and decompresses the matching entry to memory. Supports store and deflate.
• replaceBinary branches on builtin.os.tag. On Windows it now:
  1. Writes the new bytes to <install>.new.
  2. MoveFileExW(install, install + ".old", REPLACE_EXISTING) — rename of a running .exe is allowed on NTFS.
  3. MoveFileExW(install + ".new", install, REPLACE_EXISTING) — promote.
  • On failure of step 3, rolls back by renaming .old back to install.
• cleanupStaleBinary runs at the top of main() and deletes <exe>.old from a previous upgrade (no-op on non-Windows). The currently-running process can't delete its own .old, so cleanup happens on the next launch.
• doUpgrade resolves install_path before extraction so the zip path can use its parent directory as scratch space for the temp .zip.

Notes

• MoveFileExW is extern-declared locally — std.os.windows doesn't expose it in 0.16.
• std.Io.Dir.renameAbsolute was not used for the running-exe rename: it opens the source with GENERIC_WRITE, which conflicts with the loader's FILE_SHARE_READ | FILE_SHARE_DELETE sharing on a running .exe. MoveFileExW only needs DELETE.
• MOVEFILE_DELAY_UNTIL_REBOOT was not used as a fallback because it writes to HKLM\...\PendingFileRenameOperations which requires admin. Cleanup at next-launch is the primary mechanism.

Test plan

• zig build test passes on Linux (existing tar.gz tests + new in-memory zip extraction tests using a hand-rolled stored-zip fixture).
• zig build -Dtarget=x86_64-windows-gnu produces zemo.exe (verified file output: PE32+ executable for MS Windows).
• Manual smoke test on Windows x86_64: built a VERSION="0.2.9" variant of this branch, replaced the installed 0.3.0 binary on a real Windows host, then ran the full upgrade flow against the existing v0.3.0 GitHub release.
  • zemo upgrade --check reports Newer version available: v0.2.9 -> v0.3.0.
  • zemo upgrade downloads zemo-x86_64-windows.zip, extracts zemo.exe, replaces the running binary.
  • Subsequent zemo --version shows 0.3.0.
  • No leftover <exe>.old after the next zemo invocation (deleted by cleanupStaleBinary on startup).