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/hud_plugin.rs

@@ -1741,6 +1741,7 @@ fn update_hud(
                 GameMode::Zen => "ZEN".to_string(),
                 GameMode::Challenge => "CHALLENGE".to_string(),
                 GameMode::TimeAttack => "TIME ATTACK".to_string(),
+                GameMode::Difficulty(level) => level.label().to_uppercase(),
             };
         }