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/stage.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+set -eu
+
+prefix=${1:?installation prefix required}
+binary=${2:-target/release/war_in_tunnels}
+install -Dm755 "$binary" "$prefix/lib/war-in-tunnels/war-in-tunnels"
+mkdir -p "$prefix/bin"
+cat > "$prefix/bin/war-in-tunnels" <<'EOF'
+#!/bin/sh
+set -eu
+bindir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+exec "$bindir/../lib/war-in-tunnels/war-in-tunnels" "$@"
+EOF
+chmod +x "$prefix/bin/war-in-tunnels"
+cp -r soundtracks "$prefix/lib/war-in-tunnels/"
+install -Dm644 LICENSE "$prefix/share/licenses/war-in-tunnels/LICENSE"
+install -Dm644 packaging/com.garandplg.WarInTunnels.desktop "$prefix/share/applications/com.garandplg.WarInTunnels.desktop"
+install -Dm644 packaging/com.garandplg.WarInTunnels.svg "$prefix/share/icons/hicolor/scalable/apps/com.garandplg.WarInTunnels.svg"