feat(engine): theme registry + discovery (Card theme phase 6)

Implements Phase 6 of CARD_PLAN.md — discovers every available card
theme on startup so the future picker UI can list them.

solitaire_engine/src/theme/registry.rs
  ThemeEntry { id, display_name, manifest_url, meta }
  ThemeRegistry — Resource holding the entries; provides
    find(id), iter(), len(), is_empty().
  ThemeRegistryPlugin — Startup system that scans
    user_theme_dir() and populates the registry.
  build_registry(user_dir) — pure helper; takes the dir as a
    parameter so tests use tempfile::tempdir() without touching
    the global OnceLock-based user-theme path.
  refresh_registry(&mut, user_dir) — replaces in-place; called
    after a successful import_theme so a freshly-imported theme
    appears in the picker without an app restart.

The bundled default entry is always inserted (id "default", served
from DEFAULT_THEME_MANIFEST_URL) so the picker has at least one
option even when no user themes exist.

Discovery is best-effort: a directory whose theme.ron is missing,
malformed, or fails ThemeMeta::validate is silently skipped — broken
themes don't poison the registry. Only the meta block is parsed
(via a derive(Deserialize) struct that ignores other manifest
fields), which keeps startup quick even with dozens of themes
installed.

Wired into solitaire_app/main.rs after ThemePlugin so the asset
sources are registered before discovery scans for theme.ron files.

10 new tests covering: empty user dir, nonexistent user dir, valid
user theme registers, full-manifest tolerance via meta-only parser,
malformed theme.ron skipped, invalid-meta theme skipped, directory
without theme.ron ignored, find() returns None for unknown id,
refresh_registry replaces stale entries, default-entry URL matches
the embedded constant.

cargo build / clippy --workspace --all-targets -- -D warnings / test
--workspace all green (960 passed, 0 failed, 9 ignored).

solitaire_engine/src/lib.rs

@@ -44,7 +44,10 @@ pub use assets::{
     populate_embedded_default_theme, register_theme_asset_sources, AssetSourcesPlugin,
     DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
 };
-pub use theme::{set_theme, ActiveTheme, CardTheme, CardThemeLoader, ThemePlugin};
+pub use theme::{
+    set_theme, ActiveTheme, CardTheme, CardThemeLoader, ThemeEntry, ThemePlugin, ThemeRegistry,
+    ThemeRegistryPlugin,
+};
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource, AchievementsScreen};
 pub use challenge_plugin::{
     challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,