feat(engine,core,data): add Challenge mode with seed list and level-5 gate

Phase 6 part 4b (partial):
- GameMode::Challenge variant in solitaire_core. undo() returns
  RuleViolation when mode is Challenge so the player commits to each
  decision.
- solitaire_data::challenge defines a stable CHALLENGE_SEEDS list with
  challenge_seed_for(index) wrapping modulo length.
- PlayerProgress.challenge_index (serde-default for older saves) tracks
  how far the player has progressed.
- ChallengePlugin advances the cursor on Challenge-mode wins, persists,
  and emits ChallengeAdvancedEvent. Pressing X starts a Challenge-mode
  game with the current seed; gated to level >= CHALLENGE_UNLOCK_LEVEL (5).
- InputPlugin's Z key (Zen mode) is now also gated to level >= 5.

Time Attack and unlock UI still deferred.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_app/src/main.rs

@@ -1,7 +1,7 @@
 use bevy::prelude::*;
 use solitaire_engine::{
-    AchievementPlugin, AnimationPlugin, CardPlugin, DailyChallengePlugin, GamePlugin, InputPlugin,
-    ProgressPlugin, StatsPlugin, TablePlugin, WeeklyGoalsPlugin,
+    AchievementPlugin, AnimationPlugin, CardPlugin, ChallengePlugin, DailyChallengePlugin,
+    GamePlugin, InputPlugin, ProgressPlugin, StatsPlugin, TablePlugin, WeeklyGoalsPlugin,
 };
 
 fn main() {
@@ -26,5 +26,6 @@ fn main() {
         .add_plugins(AchievementPlugin::default())
         .add_plugins(DailyChallengePlugin)
         .add_plugins(WeeklyGoalsPlugin)
+        .add_plugins(ChallengePlugin)
         .run();
 }