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.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
+  <rect width="128" height="128" rx="16" fill="#20252b"/>
+  <path d="M20 104V56a44 44 0 0 1 88 0v48H88V56a24 24 0 0 0-48 0v48Z" fill="#bd985e"/>
+  <path d="M51 91V59l13 15 13-15v32" fill="none" stroke="#e9edf0" stroke-width="7"/>
+</svg>