Add apt install instructions for clang, lld, llvm, and nsis to README Ubuntu-to-Windows build section. Add clang-cl availability check to cargo-xwin wrapper script with helpful error message directing users to install LLVM/Clang toolchain when missing.
Add cargo-xwin wrapper script that invokes `cargo xwin` subcommand and update Linux Windows build script to use wrapper as --runner instead of cargo-xwin binary name directly. Add chmod +x to ensure wrapper is executable before build.
