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_engine/src/lib.rs

@@ -40,6 +40,10 @@ pub mod ui_tooltip;
 pub mod weekly_goals_plugin;
 pub mod win_summary_plugin;
 
+pub use assets::{
+    populate_embedded_default_theme, register_theme_asset_sources, AssetSourcesPlugin,
+    DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
+};
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource, AchievementsScreen};
 pub use challenge_plugin::{
     challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,