The harness defaulted every case to a linux platform mock while extracting
onto the real filesystem, so on Windows the install always ended 'incomplete':
the runtime checks POSIX exec bits unless the platform is win32, and NTFS
never sets them. Default the harness platform to the host and let the
release fixture (archive, executable and harness names) follow. The install
mechanics under test are platform-neutral; the two tests that pin a
platform explicitly still do.

Co-Authored-By: Claude Code <noreply@anthropic.com>