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/events.rs

@@ -179,6 +179,16 @@ pub struct StartDailyChallengeRequestEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct StartPlayBySeedRequestEvent;
 
+/// Request to start a game at a specific difficulty tier. Fired by the
+/// difficulty section in the home overlay. The handler in `difficulty_plugin`
+/// picks a seed from the corresponding pre-verified catalog (or generates a
+/// random system-time seed for `DifficultyLevel::Random`) and writes a
+/// `NewGameRequestEvent`.
+#[derive(Message, Debug, Clone, Copy)]
+pub struct StartDifficultyRequestEvent {
+    pub level: solitaire_core::game_state::DifficultyLevel,
+}
+
 /// Request to toggle the Stats overlay. Fired by the HUD Menu-popover
 /// "Stats" row alongside the existing `S` accelerator.
 #[derive(Message, Debug, Clone, Copy, Default)]