feat(engine): asset sources for embedded + user theme dirs (Card theme phase 3)

Implements Phase 3 of CARD_PLAN.md — the embedded:// + themes:// asset
sources the card-theme system loads from. The bundled default-theme
manifest ships in the binary via Bevy's EmbeddedAssetRegistry; user
themes load from user_theme_dir() through a FileAssetReader-backed
source registered as `themes://`.

Registration is split across:
  register_theme_asset_sources(&mut App)
    Called BEFORE DefaultPlugins. Registers `themes://` while
    AssetSourceBuilders is still mutable.
  AssetSourcesPlugin
    Added AFTER DefaultPlugins. Populates the EmbeddedAssetRegistry
    that AssetPlugin's build step would otherwise overwrite.

Constants exposed for downstream consumers:
  USER_THEMES                 = "themes"   (asset-source name)
  DEFAULT_THEME_MANIFEST_URL  = "embedded://solitaire_engine/assets/themes/default/theme.ron"

Includes a stub default theme.ron (52 face slots + back) so
`ThemeManifest::validate()` accepts it today; PROVENANCE.md documents
the plan to drop in real SVG art (hayeah/playing-cards-assets) in a
follow-up.

4 new tests covering source registration, embedded-registry
population, manifest validation against the embedded stub, and the
manifest-URL constant matching the embedded asset path.

cargo check --workspace --all-targets / clippy --workspace
--all-targets -- -D warnings clean. cargo test could not be run in
this turn because the C linker (cc) is unexpectedly absent from the
sandbox; the test bodies compile cleanly under cargo check --tests
and will run on a normal toolchain.

solitaire_app/src/main.rs

@@ -6,12 +6,13 @@ use bevy::prelude::*;
 use bevy::window::{MonitorSelection, PresentMode, WindowPosition};
 use solitaire_data::{load_settings_from, provider_for_backend, settings_file_path, Settings};
 use solitaire_engine::{
-    AchievementPlugin, AnimationPlugin, AudioPlugin, AutoCompletePlugin, CardAnimationPlugin,
-    CardPlugin, ChallengePlugin, CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin,
-    FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
-    OnboardingPlugin, PausePlugin, ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin,
-    SplashPlugin, StatsPlugin, SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin,
-    UiModalPlugin, UiTooltipPlugin, WeeklyGoalsPlugin, WinSummaryPlugin,
+    register_theme_asset_sources, AchievementPlugin, AnimationPlugin, AssetSourcesPlugin,
+    AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
+    CursorPlugin, DailyChallengePlugin, FeedbackAnimPlugin, FontPlugin, GamePlugin, HelpPlugin,
+    HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin, OnboardingPlugin, PausePlugin,
+    ProfilePlugin, ProgressPlugin, SelectionPlugin, SettingsPlugin, SplashPlugin, StatsPlugin,
+    SyncPlugin, TablePlugin, TimeAttackPlugin, UiFocusPlugin, UiModalPlugin, UiTooltipPlugin,
+    WeeklyGoalsPlugin, WinSummaryPlugin,
 };
 
 fn main() {
@@ -54,7 +55,17 @@ fn main() {
         ),
     };
 
-    App::new()
+    let mut app = App::new();
+
+    // The card-theme system's `themes://` asset source must be
+    // registered *before* `DefaultPlugins` builds `AssetPlugin`,
+    // because that plugin freezes the asset-source list at build
+    // time. The matching `AssetSourcesPlugin` (added below) finishes
+    // the wiring after `DefaultPlugins` by populating the embedded
+    // default theme into Bevy's `EmbeddedAssetRegistry`.
+    register_theme_asset_sources(&mut app);
+
+    app
         .add_plugins(
             DefaultPlugins
                 .set(WindowPlugin {
@@ -91,6 +102,7 @@ fn main() {
                     ..default()
                 }),
         )
+        .add_plugins(AssetSourcesPlugin)
         .add_plugins(FontPlugin)
         .add_plugins(GamePlugin)
         .add_plugins(TablePlugin)