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.

.gitignore

@@ -1,4 +1,5 @@
 /target
 /result
+/dist
 
 war_in_tunnels.log