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_data/src/stats.rs

@@ -104,6 +104,13 @@ impl StatsExt for StatsSnapshot {
             // Time Attack uses its own session-level scoring; a per-game best
             // wouldn't compose with the other modes' single-game numbers.
             GameMode::TimeAttack => {}
+            // Difficulty games pool into the Classic best-score/time buckets per
+            // the user's stats preference.
+            GameMode::Difficulty(_) => {
+                self.classic_best_score = self.classic_best_score.max(score_u32);
+                self.classic_fastest_win_seconds =
+                    min_ignore_zero(self.classic_fastest_win_seconds, time_seconds);
+            }
         }
         self.last_modified = Utc::now();
     }