Replaces linux/windows/macos-testing-build.yml with a single testing-build.yml
(matrix over ubuntu/windows/macos) that publishes all artifacts to one
0.0.0-testing-<sha> prerelease. Manual dispatch + 0.0.0-testing-* tag push.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>