Add Linux packaging workflow and related scripts

Introduce a new CI workflow `.gitea/workflows/packages.yml` that builds
deb, rpm, AppImage, Flatpak and AUR packages on push and PR events.
Add packaging helpers (`build.sh`, `stage.sh`, spec file, PKGBUILD,
desktop entry, SVG icon) and supporting assets under `packaging/`.
Update `.gitignore` to ignore the generated `dist` directory.
Expand documentation (CODE_MAP, AGENTS, PROGRESS, TESTING and new
PACKAGING.md) to describe the packaging process and its limitations.
Adjust `src/app/threads/audio.rs` and `tests/audio.rs` imports to
support loading audio files from installed package layouts.

packaging/com.garandplg.WarInTunnels.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=War in Tunnels
+Comment=Underground strategy game in your terminal
+Exec=war-in-tunnels
+Icon=com.garandplg.WarInTunnels
+Terminal=true
+Categories=Game;StrategyGame;