Package the models on every platform, not just the phone

The Android bundling landed the weights under that platform's asset directory,
which was the wrong home the moment a second packager wanted them. `makepkg -si`
produced a desktop install with no model at all — the same "no face model is
installed" the phone used to show, for the same reason: nothing put the files
anywhere the app looks.

So `models/face/` at the root is the one copy, and both packagers read it:
assemble-apk.sh bundles it as APK assets, and the PKGBUILD installs it to
/usr/share/darkroom/models. Both refuse an LFS pointer rather than shipping a
130-byte file that fails inside the graph loader on a user's machine.

`face_models` now searches three places, most specific first: the account's own
directory, the shared user directory, then $XDG_DATA_DIRS. So a packaged pair is
found automatically and a pair the user placed by hand still outranks it — which
is what keeps a deliberate choice of weights from being overridden by an
upgrade.

$XDG_DATA_DIRS rather than a hard-coded /usr/share: that is the variable a
distribution, a prefix install or a Nix-style store already sets to say where
its data went, and its documented default is exactly the two paths that would
otherwise have been hard-coded. Empty on Android, which has no such directories
— there the APK's copy is unpacked into the shared user directory instead,
because an asset inside a package is not a path anything can read from.

Verified: the APK still carries both models at assets/models/, the PKGBUILD
parses and installs from the new path, 467 tests pass.

Includes the pkgver 0.6.0 → 0.7.0 bump that was already sitting uncommitted in
the working tree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/library.rs

@@ -2934,17 +2934,50 @@ pub fn shared_face_models_dir() -> PathBuf {
 /// `tools/fix-face-model-shapes.sh` has to run over the originals first,
 /// because tract cannot parse either graph with a dynamic input.
 ///
-/// The per-account directory wins over the shared one so a library can be
-/// pinned to its own weights — a re-index after a model swap is a `model_id`
-/// change, and someone mid-migration needs the old pair to stay put for one
-/// account without holding back the other.
+/// Searched in three places, most specific first:
+///
+/// 1. **The account's own directory.** A library can be pinned to its own
+///    weights — a model swap is a `model_id` change and a re-index, and someone
+///    mid-migration needs one account's pair to stay put without holding the
+///    other back.
+/// 2. **The shared user directory.** Where a user drops a pair by hand, and
+///    where the Android entry point unpacks the copy the APK carries.
+/// 3. **The system directories.** Where a package installs them — the Arch
+///    package puts the pair in `/usr/share/darkroom/models`. Last, so anything
+///    the user placed themselves outranks what the package shipped.
 pub fn face_models(server: &str, user_id: &str) -> Option<(PathBuf, PathBuf)> {
     fn pair(dir: PathBuf) -> Option<(PathBuf, PathBuf)> {
         let detector = dir.join("scrfd_500m_640.onnx");
         let embedder = dir.join("arcface_mbf_b1.onnx");
         (detector.is_file() && embedder.is_file()).then_some((detector, embedder))
     }
-    pair(face_models_dir(server, user_id)).or_else(|| pair(shared_face_models_dir()))
+    pair(face_models_dir(server, user_id))
+        .or_else(|| pair(shared_face_models_dir()))
+        .or_else(|| system_face_models_dirs().into_iter().find_map(pair))
+}
+
+/// Where a *package* may have installed the models.
+///
+/// `$XDG_DATA_DIRS` rather than a hard-coded `/usr/share`, because that is the
+/// variable a distribution, a prefix install, or a Nix-style store sets to say
+/// where its data went, and the default it falls back to is exactly the pair of
+/// paths that would otherwise have been hard-coded.
+///
+/// Empty on Android, which has no such directories: there the APK's copy is
+/// unpacked into the shared user directory instead, because an asset inside a
+/// package is not a path anything can read from (ARCH §6.9).
+fn system_face_models_dirs() -> Vec<PathBuf> {
+    if cfg!(target_os = "android") {
+        return Vec::new();
+    }
+    let dirs = std::env::var("XDG_DATA_DIRS")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .unwrap_or_else(|| "/usr/local/share:/usr/share".into());
+    dirs.split(':')
+        .filter(|d| !d.is_empty())
+        .map(|d| PathBuf::from(d).join("darkroom").join("models"))
+        .collect()
 }
 
 /// One grid cell's data, read from the catalog.