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/aur/PKGBUILD

@@ -0,0 +1,28 @@
+# Test recipe: CI supplies the source archive from the checked-out commit.
+pkgname=war-in-tunnels
+pkgver=0.1.0
+pkgrel=1
+pkgdesc='Underground strategy game in your terminal'
+arch=('x86_64')
+url='https://gitea.garandplg.com/GarandPLG/war-in-tunnels'
+license=('MIT')
+depends=('alsa-lib' 'gcc-libs' 'glibc')
+makedepends=('cargo' 'pkgconf')
+options=('!debug')
+source=("$pkgname-$pkgver.tar.gz")
+sha256sums=('SKIP')
+
+build() {
+  cd "$srcdir/$pkgname-$pkgver"
+  cargo build --release --locked
+}
+
+check() {
+  cd "$srcdir/$pkgname-$pkgver"
+  cargo test --release --locked
+}
+
+package() {
+  cd "$srcdir/$pkgname-$pkgver"
+  sh packaging/stage.sh "$pkgdir/usr"
+}