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_data/src/lib.rs

@@ -57,3 +57,6 @@ pub use weekly::{
     current_iso_week_key, weekly_goal_by_id, WeeklyGoalContext, WeeklyGoalDef, WeeklyGoalKind,
     WEEKLY_GOALS, WEEKLY_GOAL_XP,
 };
+
+pub mod challenge;
+pub use challenge::{challenge_count, challenge_seed_for, CHALLENGE_SEEDS};