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_engine/src/lib.rs

@@ -3,6 +3,7 @@
 pub mod achievement_plugin;
 pub mod animation_plugin;
 pub mod card_plugin;
+pub mod challenge_plugin;
 pub mod daily_challenge_plugin;
 pub mod events;
 pub mod game_plugin;
@@ -15,6 +16,9 @@ pub mod table_plugin;
 pub mod weekly_goals_plugin;
 
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource};
+pub use challenge_plugin::{
+    challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,
+};
 pub use daily_challenge_plugin::{
     DailyChallengeCompletedEvent, DailyChallengePlugin, DailyChallengeResource,
 };