fix(engine): unify light/dark theme manifest URL resolution

- set_theme resolved every id to themes:// — the bundled dark/classic
  themes live at embedded://, so switching to them via the public API
  would NotFound and silently keep the old theme. All three load paths
  now share one theme_manifest_url resolver.
- load_initial_theme's settings-absent fallback was "classic", a
  leftover from v0.33 when classic was the default; it now derives from
  Settings::default() so it can't drift from the data crate again, with
  a test pinning the default id to a bundled theme.
- settings.rs doc no longer claims "classic" is migrated to "dark";
  only the pre-rename "default" id is rewritten.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/theme/plugin.rs

@@ -116,24 +116,32 @@ impl Plugin for ThemePlugin {
     }
 }
 
+/// Resolves the manifest URL for `theme_id`: the bundled themes
+/// (`"dark"`, `"classic"`) load from the embedded source; any other id
+/// is a user theme served from the `themes://` directory source.
+fn theme_manifest_url(theme_id: &str) -> String {
+    bundled_theme_url(theme_id)
+        .map(str::to_string)
+        .unwrap_or_else(|| format!("themes://{theme_id}/theme.ron"))
+}
+
 /// Kicks off the initial theme load — the one named by
 /// `Settings::selected_theme_id` if available, falling back to the
-/// embedded default. The actual rasterisation runs asynchronously on
-/// the asset task pool; the sync system below picks up the
-/// `LoadedWithDependencies` event when every face + back is ready.
+/// fresh-install default from `solitaire_data::Settings`. The actual
+/// rasterisation runs asynchronously on the asset task pool; the sync
+/// system below picks up the `LoadedWithDependencies` event when every
+/// face + back is ready.
 fn load_initial_theme(
     asset_server: Res<AssetServer>,
     settings: Option<Res<crate::settings_plugin::SettingsResource>>,
     mut commands: Commands,
 ) {
+    let default_id = solitaire_data::Settings::default().selected_theme_id;
     let id = settings
         .as_deref()
         .map(|s| s.0.selected_theme_id.as_str())
-        .unwrap_or("classic");
-    let url = bundled_theme_url(id)
-        .map(str::to_string)
-        .unwrap_or_else(|| format!("themes://{id}/theme.ron"));
-    let handle: Handle<CardTheme> = asset_server.load(url);
+        .unwrap_or(&default_id);
+    let handle: Handle<CardTheme> = asset_server.load(theme_manifest_url(id));
     commands.insert_resource(ActiveTheme(handle));
 }
 
@@ -162,10 +170,7 @@ fn react_to_settings_theme_change(
         return;
     }
 
-    let url = bundled_theme_url(new_id)
-        .map(str::to_string)
-        .unwrap_or_else(|| format!("themes://{new_id}/theme.ron"));
-    let handle: Handle<CardTheme> = asset_server.load(url);
+    let handle: Handle<CardTheme> = asset_server.load(theme_manifest_url(new_id));
     commands.insert_resource(ActiveTheme(handle));
 }
 
@@ -252,10 +257,11 @@ fn apply_theme_to_card_image_set(theme: &CardTheme, image_set: &mut CardImageSet
     image_set.theme_back = Some(theme.back.clone());
 }
 
-/// Switches the active theme to the one served at
-/// `themes://<theme_id>/theme.ron`. Returns the new `Handle<CardTheme>`
-/// so callers can poll `Assets<CardTheme>` if they want to wait for
-/// the load before changing UI state.
+/// Switches the active theme to `theme_id` — the embedded source for
+/// the bundled `"dark"` / `"classic"` themes, `themes://` for user
+/// themes. Returns the new `Handle<CardTheme>` so callers can poll
+/// `Assets<CardTheme>` if they want to wait for the load before
+/// changing UI state.
 ///
 /// The handle is also written to the [`ActiveTheme`] resource — the
 /// per-frame sync system picks up the `LoadedWithDependencies` event
@@ -266,8 +272,7 @@ pub fn set_theme(
     asset_server: &AssetServer,
     theme_id: &str,
 ) -> Handle<CardTheme> {
-    let url = format!("themes://{theme_id}/theme.ron");
-    let handle: Handle<CardTheme> = asset_server.load(url);
+    let handle: Handle<CardTheme> = asset_server.load(theme_manifest_url(theme_id));
     commands.insert_resource(ActiveTheme(handle.clone()));
     handle
 }
@@ -475,13 +480,43 @@ mod tests {
     }
 
     #[test]
-    fn set_theme_url_format_matches_themes_source() {
-        // The format string is the only behavioural surface of
-        // set_theme that doesn't require an App. We assert the URL
-        // shape so a future refactor doesn't accidentally change the
-        // path layout.
-        let url = format!("themes://{}/theme.ron", "user_uploaded");
-        assert_eq!(url, "themes://user_uploaded/theme.ron");
+    fn theme_manifest_url_routes_bundled_ids_to_embedded_source() {
+        // The bundled themes live in the binary, not in the user
+        // themes directory — resolving them to `themes://` would
+        // NotFound and silently leave the previous theme active.
+        assert_eq!(
+            theme_manifest_url("dark"),
+            crate::assets::DARK_THEME_MANIFEST_URL
+        );
+        assert_eq!(
+            theme_manifest_url("classic"),
+            crate::assets::CLASSIC_THEME_MANIFEST_URL
+        );
+    }
+
+    #[test]
+    fn theme_manifest_url_routes_user_ids_to_themes_source() {
+        // We assert the URL shape so a future refactor doesn't
+        // accidentally change the path layout of the user-themes
+        // source.
+        assert_eq!(
+            theme_manifest_url("user_uploaded"),
+            "themes://user_uploaded/theme.ron"
+        );
+    }
+
+    #[test]
+    fn initial_theme_fallback_matches_settings_default() {
+        // `load_initial_theme` falls back to the data-crate default
+        // when `SettingsResource` is absent. That default must always
+        // name a bundled theme, or a fresh minimal setup would resolve
+        // to a nonexistent user theme (see the v0.33.0 "classic"
+        // fallback drift in CHANGELOG.md).
+        let default_id = solitaire_data::Settings::default().selected_theme_id;
+        assert!(
+            bundled_theme_url(&default_id).is_some(),
+            "default theme id {default_id:?} must be a bundled theme"
+        );
     }
 
     /// Test 1: the bundled dark theme always has embedded SVG bytes