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/input_plugin.rs

@@ -24,10 +24,12 @@ use solitaire_core::pile::PileType;
 use solitaire_core::rules::{can_place_on_foundation, can_place_on_tableau};
 
 use crate::card_plugin::{CardEntity, TABLEAU_FAN_FRAC};
+use crate::challenge_plugin::CHALLENGE_UNLOCK_LEVEL;
 use crate::events::{
     DrawRequestEvent, MoveRequestEvent, NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
 };
 use crate::game_plugin::GameMutation;
+use crate::progress_plugin::ProgressResource;
 use crate::layout::{Layout, LayoutResource};
 use crate::resources::{DragState, GameStateResource};
 
@@ -61,6 +63,7 @@ impl Plugin for InputPlugin {
 
 fn handle_keyboard(
     keys: Res<ButtonInput<KeyCode>>,
+    progress: Option<Res<ProgressResource>>,
     mut undo: EventWriter<UndoRequestEvent>,
     mut new_game: EventWriter<NewGameRequestEvent>,
     mut draw: EventWriter<DrawRequestEvent>,
@@ -72,10 +75,20 @@ fn handle_keyboard(
         new_game.send(NewGameRequestEvent::default());
     }
     if keys.just_pressed(KeyCode::KeyZ) {
-        new_game.send(NewGameRequestEvent {
-            seed: None,
-            mode: Some(solitaire_core::game_state::GameMode::Zen),
-        });
+        // Zen / Challenge / Time Attack are gated to level >= CHALLENGE_UNLOCK_LEVEL.
+        // X is gated separately by ChallengePlugin.
+        let level = progress.as_ref().map_or(0, |p| p.0.level);
+        if level >= CHALLENGE_UNLOCK_LEVEL {
+            new_game.send(NewGameRequestEvent {
+                seed: None,
+                mode: Some(solitaire_core::game_state::GameMode::Zen),
+            });
+        } else {
+            info!(
+                "Zen mode locked — reach level {} (currently {}).",
+                CHALLENGE_UNLOCK_LEVEL, level
+            );
+        }
     }
     if keys.just_pressed(KeyCode::KeyD) {
         draw.send(DrawRequestEvent);