refactor/modules
master
main.rs keeps only the CLI definition and command dispatch: - registry.rs: image-name validation + XDG registry path handling (tests) - build.rs: podman mount/unmount, vmlinuz extraction, cpio/gzip initrd - qemu.rs: direct-kernel-boot VM launch No behavior change; cargo fmt/clippy/test pass, build/run paths verified against the mock-vm image.