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/challenge.rs

@@ -0,0 +1,73 @@
+//! Static seed list for Challenge mode + helpers.
+//!
+//! Challenge mode walks a fixed sequence of hard-but-winnable seeds. The
+//! player advances by winning a deal in `GameMode::Challenge`. The
+//! `challenge_index` cursor is stored per-player in `PlayerProgress`.
+//!
+//! Seeds wrap modulo `CHALLENGE_SEEDS.len()` so a sufficiently dedicated
+//! player never runs out of challenges.
+
+/// Curated Challenge-mode seeds. Order is stable across versions; add new
+/// seeds at the end.
+pub const CHALLENGE_SEEDS: &[u64] = &[
+    0xDEAD_BEEF_CAFE_F00D,
+    0xC0DE_FACE_8BAD_F00D,
+    0xFEE1_DEAD_DEAD_BEEF,
+    0xBAAD_F00D_BAAD_F00D,
+    0x1337_C0DE_4242_BABE,
+];
+
+/// Resolve a `challenge_index` to its corresponding seed, wrapping when
+/// the index exceeds the seed-list length. Returns `None` if the seed list
+/// is empty (defensive — `CHALLENGE_SEEDS` is non-empty by construction).
+pub fn challenge_seed_for(index: u32) -> Option<u64> {
+    if CHALLENGE_SEEDS.is_empty() {
+        return None;
+    }
+    Some(CHALLENGE_SEEDS[(index as usize) % CHALLENGE_SEEDS.len()])
+}
+
+/// Total number of currently-defined challenges. Useful for displaying
+/// "Challenge {n + 1} of {total}" in UI.
+pub fn challenge_count() -> u32 {
+    CHALLENGE_SEEDS.len() as u32
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn challenge_seed_for_0_is_first_seed() {
+        assert_eq!(challenge_seed_for(0), Some(CHALLENGE_SEEDS[0]));
+    }
+
+    #[test]
+    fn challenge_seed_wraps_past_end() {
+        let len = CHALLENGE_SEEDS.len() as u32;
+        assert_eq!(
+            challenge_seed_for(len),
+            Some(CHALLENGE_SEEDS[0]),
+            "wraps to seed 0 when index == len"
+        );
+        assert_eq!(
+            challenge_seed_for(len + 2),
+            Some(CHALLENGE_SEEDS[2]),
+            "wraps modulo len"
+        );
+    }
+
+    #[test]
+    fn all_challenge_seeds_are_unique() {
+        let mut seeds: Vec<u64> = CHALLENGE_SEEDS.to_vec();
+        seeds.sort();
+        let len_before = seeds.len();
+        seeds.dedup();
+        assert_eq!(seeds.len(), len_before);
+    }
+
+    #[test]
+    fn challenge_count_matches_seed_list_length() {
+        assert_eq!(challenge_count() as usize, CHALLENGE_SEEDS.len());
+    }
+}