test(sync): add unit tests for StatsSnapshot::win_rate and AchievementRecord::unlock

Both public APIs in solitaire_sync had no test coverage:
- win_rate(): None before any game, 100/50/0% cases
- AchievementRecord::locked(), unlock(), idempotency preserving earliest date

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_sync/src/achievements.rs

@@ -46,3 +46,36 @@ impl AchievementRecord {
         self.unlock_date = Some(at);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn locked_creates_an_unlocked_record() {
+        let r = AchievementRecord::locked("first_win");
+        assert_eq!(r.id, "first_win");
+        assert!(!r.unlocked);
+        assert!(r.unlock_date.is_none());
+        assert!(!r.reward_granted);
+    }
+
+    #[test]
+    fn unlock_sets_unlocked_and_stores_timestamp() {
+        let mut r = AchievementRecord::locked("first_win");
+        let ts = Utc::now();
+        r.unlock(ts);
+        assert!(r.unlocked);
+        assert_eq!(r.unlock_date, Some(ts));
+    }
+
+    #[test]
+    fn unlock_is_idempotent_and_preserves_earliest_date() {
+        let mut r = AchievementRecord::locked("first_win");
+        let early = DateTime::UNIX_EPOCH;
+        let later = Utc::now();
+        r.unlock(early);
+        r.unlock(later); // should be a no-op
+        assert_eq!(r.unlock_date, Some(early), "earliest unlock date must be preserved");
+    }
+}

solitaire_sync/src/stats.rs

@@ -74,3 +74,53 @@ impl StatsSnapshot {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn win_rate_is_none_before_any_game() {
+        let s = StatsSnapshot::default();
+        assert!(s.win_rate().is_none());
+    }
+
+    #[test]
+    fn win_rate_100_when_all_games_won() {
+        let s = StatsSnapshot {
+            games_played: 5,
+            games_won: 5,
+            ..StatsSnapshot::default()
+        };
+        let rate = s.win_rate().expect("should have a rate");
+        assert!((rate - 100.0).abs() < 0.01, "expected 100.0, got {rate}");
+    }
+
+    #[test]
+    fn win_rate_50_when_half_won() {
+        let s = StatsSnapshot {
+            games_played: 10,
+            games_won: 5,
+            ..StatsSnapshot::default()
+        };
+        let rate = s.win_rate().expect("should have a rate");
+        assert!((rate - 50.0).abs() < 0.01, "expected 50.0, got {rate}");
+    }
+
+    #[test]
+    fn win_rate_0_when_no_wins() {
+        let s = StatsSnapshot {
+            games_played: 3,
+            games_won: 0,
+            ..StatsSnapshot::default()
+        };
+        let rate = s.win_rate().expect("should have a rate");
+        assert!((rate - 0.0).abs() < 0.01, "expected 0.0, got {rate}");
+    }
+
+    #[test]
+    fn fastest_win_seconds_defaults_to_max() {
+        let s = StatsSnapshot::default();
+        assert_eq!(s.fastest_win_seconds, u64::MAX);
+    }
+}