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_engine/src/win_summary_plugin.rs

@@ -352,7 +352,7 @@ impl ScoreBreakdown {
         let no_undo_bonus = if undo_count == 0 { SCORE_NO_UNDO_BONUS } else { 0 };
         let multiplier = match mode {
             GameMode::Zen => 0.0,
-            GameMode::Classic | GameMode::Challenge | GameMode::TimeAttack => 1.0,
+            GameMode::Classic | GameMode::Challenge | GameMode::TimeAttack | GameMode::Difficulty(_) => 1.0,
         };
         Self {
             base,
@@ -423,6 +423,7 @@ fn mode_display_name(mode: GameMode) -> &'static str {
         GameMode::Zen => "Zen",
         GameMode::Challenge => "Challenge",
         GameMode::TimeAttack => "Time Attack",
+        GameMode::Difficulty(level) => level.label(),
     }
 }