The #[cfg(target_env = "gnu")] attribute in build.rs checks the host
compiler environment, not the build target. On MSVC-hosted Rust
targeting x86_64-pc-windows-gnu, this was silently false, causing:
1. WebView2Loader static linking swap to never run
2. Resource compilation fix (windres COFF output) to never run
embed-resource finds MSVC's rc.exe via Windows SDK and produces .res
files that GNU ld can't link. Fix: detect .res format (null header)
and re-compile with MinGW windres --output-format=coff.
d26b73288d