Add comprehensive integration test workflow that exercises the action's
install, cache, and restore paths with real packages on GitHub Actions
runners. Tests cover:
- Basic install and cache round-trip (xdot, libxml2-dev)
- Cache hit and version pinning verification
- Custom apt repository support (ppa:savoury1/ffmpeg6)
- Multiple package installs with dependencies
- Support for apt-mark'd packages and version-pinned packages
- ARM64 architecture support
- Package with conflicts (default-jdk replacing default-jre-headless)
- Invalid inputs (bad version, empty packages, bad repo)
- dpkg registration verification after cache restore
Co-developed-by: Claude Code v2.1.58 (claude-opus-4-6)
