Windows rustc response and host tool fixes

The prelude now writes rustc response files in the rustc-response format, which keeps arguments verbatim so Windows does not strip quotes or inject backslashes. Host tool lookup on Windows also skips extensionless PATH entries, so mise shell shims no longer shadow real executables like rustc.exe. Build-script crates on Windows use a generated PowerShell wrapper instead of a Unix shell wrapper, so native rustc invocations work correctly.