fix: CL build — link shim directly to libquickjs.so, fail on error
The -L/-l linker flags weren't finding libquickjs.so in $BUILD.
Link directly instead. Also: CL build failures now exit 1 instead
of silently falling back to the C binary.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>