ci: publish verified Linux distribution packages

[U1traVeno]

Summary

• build DEB, RPM, and AppImage bundles on Ubuntu 22.04, then install and launch-test them on Ubuntu 22.04 and Fedora 43 in required PR CI
• add nightly Debian 12, Fedora latest, Arch source-build, and Rocky Linux 9 Flatpak coverage
• publish releases atomically through a verified draft with SHA-256 checksums and GitHub build provenance
• align all package versions at 0.4.2, add the GitHub MIT license template, and document Linux installation and support boundaries

Compatibility contract

• RPM: Fedora 43+
• DEB: Ubuntu 22.04+ and Debian 12+
• Flatpak bundle: tested on Rocky Linux 9 and intended for RHEL 9-compatible systems
• AppImage: best-effort convenience artifact
• Arch: source recipe attached to releases; no AUR publication while account registration is unavailable

Tauri 2 requires WebKitGTK 4.1, which RHEL 9 does not provide in its official repositories. The native RPM therefore does not claim RHEL 9 support; the Flatpak runtime is the RHEL 9 path.

Verification

• npm test (104 tests)
• npm run build
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked (15 tests)
• local macOS DMG build at version 0.4.2
• actionlint and ShellCheck

Refs #34