The first cut derived the package from `binding-${platform}-${arch}` with an
exact-suffix match, which never matches Windows (`-msvc`) or Linux
(`-gnu`/`-musl`) names, so the repair only ever worked on macOS and
install.sh had to gate it there. Rolldown's own loader already resolves
platform, arch and libc and prints the exact `@rolldown/binding-*` it wanted
in its error chain; parse that instead and drop the gate. Also spawn npm
through a shell on Windows (Node refuses to spawn npm.cmd directly) and trim
the tests to the two invariants (no-op when it loads; installs exactly what
the loader asked for, then re-probes).
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