android: the handoff — the pending rounds, the pipeline, and what is unverified

Everything written this session that has not been through the device: rounds 43
to 48 (each patch adversarially verified, 43's transient-notice expectation
fixed), a partial round 49, the pipeline scripts, the Android Auto design and
the icon comparison mock. Also the two harness changes that were never A/B'd —
adaptive waits, which may only shorten a declared wait and never lengthen it,
and a ship that runs the mock walk and the deploy at the same time. The A/B and
its 255-step baseline are in handoff/tools; main stays at what the phone has
actually walked.

handoff/rounds/ins-r51-ddir.txt

@@ -0,0 +1,41 @@
+/// The configured root, if there is one. Split out so `download_dir` and
+/// `download_dir_at` cannot drift: on the phone the second is the ONLY way
+/// the library is found at all, from a process that has no `AppHandle` to
+/// ask.
+fn configured_download_dir(cfg: &AppConfig) -> Option<PathBuf> {
+    cfg.download_dir
+        .as_deref()
+        .filter(|d| !d.is_empty())
+        .map(PathBuf::from)
+}
+
+/// Android's private library root, under the app's own data directory.
+///
+/// `<dataDir>/Library`, and `dataDir` is what Tauri's `app_data_dir()`
+/// resolves to on Android (`PathPlugin.getDataDir` → `activity.dataDir`), so
+/// on the phone it is `/data/user/0/com.soulstream.app/Library`. NOT
+/// `filesDir`, which is one level deeper — a headless open there finds no
+/// database and answers empty with nothing obviously wrong.
+#[cfg(target_os = "android")]
+fn android_library_root(data_dir: &Path) -> PathBuf {
+    data_dir.join("Library")
+}
+
+/// `download_dir`, from the app-data directory rather than an `AppHandle` —
+/// the headless Android Auto path, which runs in a process with no Tauri in
+/// it. `None` means "there is no answer without an app": on the desktop the
+/// fallback walks `audio_dir()`/`home_dir()`, which need the handle.
+pub fn download_dir_at(data_dir: &Path, cfg: &AppConfig) -> Option<PathBuf> {
+    if let Some(d) = configured_download_dir(cfg) {
+        return Some(d);
+    }
+    #[cfg(target_os = "android")]
+    let fallback = Some(android_library_root(data_dir));
+    #[cfg(not(target_os = "android"))]
+    let fallback = {
+        let _ = data_dir;
+        None
+    };
+    fallback
+}
+