ComeOnOver Desktop Launcher v1.10.21

v1.10.21: Linux MVP M4 - GitHub Actions builds linux-x64 tarball

Adds a second job to .github/workflows/release.yml that runs on
ubuntu-latest after the Windows job completes. It runs the test
suite on Linux (CI regression sentinel), publishes a self-contained
linux-x64 build, tars it excluding pdbs, and uploads to the same
GitHub Release via `gh release upload --clobber`.

Why sequential: the Windows job creates the Release via
`vpk upload github --publish`. Linux appends to it - parallel
would race release creation.

Local dry-run verified the publish output: valid ELF 64-bit
executable with +x preserved, 224 files, 44 MB compressed
tarball. Binary lives at ./ComeOnOverDesktopLauncher when the
user extracts the tarball.

Deferred to M5: AppImage/.deb packaging, Linux auto-update,
ARM64 support.

335/335 tests pass on Linux locally. Tests run on both
windows-latest AND ubuntu-latest in CI from this release onwards.