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.

tests/audio.rs

@@ -1,7 +1,9 @@
 use rodio::Decoder;
 use std::{
-  fs::read,
+  fs::{self, read},
   io::{BufReader, Cursor},
+  path::PathBuf,
+  process::{Command, Output},
 };
 use war_in_tunnels::app::threads::{SoundrackParts, Soundtrack, load_audio};
 
@@ -17,3 +19,47 @@ fn audio_repeats_after_end_of_file() {
   assert_eq!(source.by_ref().take(sample_count).count(), sample_count);
   assert_eq!(source.take(1024).count(), 1024);
 }
+
+#[test]
+fn packaged_audio_loads_without_project_working_directory() -> () {
+  if std::env::var_os("WAR_IN_TUNNELS_PACKAGED_AUDIO_TEST").is_some() {
+    let mut source: rodio::source::Amplify<
+      rodio::decoder::LoopedDecoder<BufReader<Cursor<Vec<u8>>>>,
+    > = load_audio(&Soundtrack::Default, &SoundrackParts::Calm);
+    assert_eq!(source.by_ref().take(1024).count(), 1024);
+    return;
+  }
+
+  let directory: PathBuf = std::env::temp_dir().join(format!(
+    "war-in-tunnels-packaged-audio-{}",
+    std::process::id()
+  ));
+  fs::create_dir_all(directory.join("soundtracks/default")).unwrap();
+  fs::create_dir_all(directory.join("working-directory")).unwrap();
+  let executable: PathBuf = directory.join("audio-test");
+  fs::copy(std::env::current_exe().unwrap(), &executable).unwrap();
+  for filename in ["calm.ogg", "test.ogg"] {
+    fs::copy(
+      PathBuf::from("soundtracks/default").join(filename),
+      directory.join("soundtracks/default").join(filename),
+    )
+    .unwrap();
+  }
+  let output: Output = Command::new(executable)
+    .args([
+      "--exact",
+      "packaged_audio_loads_without_project_working_directory",
+      "--nocapture",
+    ])
+    .env("WAR_IN_TUNNELS_PACKAGED_AUDIO_TEST", "1")
+    .current_dir(directory.join("working-directory"))
+    .output()
+    .unwrap();
+  fs::remove_dir_all(directory).unwrap();
+  assert!(
+    output.status.success(),
+    "{}{}",
+    String::from_utf8_lossy(&output.stdout),
+    String::from_utf8_lossy(&output.stderr)
+  );
+}