Support Flathub test refs in runtime smoke

[bhack]

Summary

• allow the Flatpak runtime smoke helper to target Flathub PR test builds
• document PyGObject as a GNOME runtime dependency, not a bundled Python dependency
• document the local screenshot-mirroring lint caveat and PR test-build smoke path

Testing

• .venv/bin/python -m pytest tests/test_check_flatpak_runtime.py -q
• .venv/bin/python -m ruff check tools/check_flatpak_runtime.py tests/test_check_flatpak_runtime.py
• .venv/bin/python -m ruff format --check tools/check_flatpak_runtime.py tests/test_check_flatpak_runtime.py
• python3 tools/check_flatpak_runtime.py --app-ref io.github.bhack.mini-eq//test