Fix Windows build: add zip pkg, portable embed (od replaces xxd)

1. Add zip to MSYS2 build packages (both dry-run and release).
2. embed-frontend.sh: replace xxd with od+sed for hex dump — xxd
   requires vim which isn't in MSYS2 CLANG64 by default.
3. embed-frontend.sh: flip IS_MACOS→IS_LINUX — Linux gets ld -r -b
   binary (ELF-only), everything else (macOS, Windows) gets the
   portable C-array approach via od.
4. release.yml: same Windows archive fix (msys2 shell + .exe detect).