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/war-in-tunnels.spec

@@ -0,0 +1,21 @@
+Name: war-in-tunnels
+Version: %{package_version}
+Release: 1
+Summary: Underground strategy game in your terminal
+License: MIT
+URL: https://gitea.garandplg.com/GarandPLG/war-in-tunnels
+Requires: alsa-lib
+
+%description
+A terminal-based underground strategy game.
+
+%install
+mkdir -p %{buildroot}
+cp -a %{staging}/. %{buildroot}/
+
+%files
+/usr/bin/war-in-tunnels
+/usr/lib/war-in-tunnels
+/usr/share/applications/com.garandplg.WarInTunnels.desktop
+/usr/share/icons/hicolor/scalable/apps/com.garandplg.WarInTunnels.svg
+%license /usr/share/licenses/war-in-tunnels/LICENSE