feat(difficulty): add difficulty-tier game mode with seed catalogs and home UI

Adds DifficultyLevel (Easy/Medium/Hard/Expert/Grandmaster/Random) to
solitaire_core::game_state alongside GameMode::Difficulty(DifficultyLevel).
Five seed catalogs (40 seeds each) are pre-verified by the new
gen_difficulty_seeds binary using tiered solver budgets (1K–200K moves).
DifficultyPlugin resolves StartDifficultyRequestEvent → catalog seed →
NewGameRequestEvent; Random uses a system-time seed and bypasses the
winnable-only filter. The home overlay gets an expandable Difficulty section
between Draw Mode and the mode grid; last-played tier persists in Settings.
Difficulty wins pool into Classic stats. 5 unit tests in difficulty_plugin.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_app/src/lib.rs

@@ -27,8 +27,8 @@ use solitaire_data::{load_settings_from, provider_for_backend, settings_file_pat
 use solitaire_engine::{
     register_theme_asset_sources, AchievementPlugin, AnimationPlugin, AssetSourcesPlugin,
     AudioPlugin, AutoCompletePlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
-    CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, FeedbackAnimPlugin, FontPlugin,
-    GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
+    CursorPlugin, DailyChallengePlugin, DiagnosticsHudPlugin, DifficultyPlugin, FeedbackAnimPlugin,
+    FontPlugin, GamePlugin, HelpPlugin, HomePlugin, HudPlugin, InputPlugin, LeaderboardPlugin,
     OnboardingPlugin, PausePlugin, PlayBySeedPlugin, ProfilePlugin, ProgressPlugin,
     RadialMenuPlugin, ReplayOverlayPlugin, ReplayPlaybackPlugin, SelectionPlugin, SettingsPlugin,
     SplashPlugin, StatsPlugin, SyncPlugin, TablePlugin, ThemePlugin, ThemeRegistryPlugin,
@@ -170,6 +170,7 @@ pub fn run() {
         .add_plugins(WeeklyGoalsPlugin)
         .add_plugins(ChallengePlugin)
         .add_plugins(PlayBySeedPlugin)
+        .add_plugins(DifficultyPlugin)
         .add_plugins(TimeAttackPlugin)
         .add_plugins(HudPlugin)
         .add_plugins(HelpPlugin)