feat(svc): ArenaTwoPickCardPoolService (rarity-weighted, class+neutral)

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

SVSim.EmulatedEntrypoint/Services/IArenaTwoPickCardPoolService.cs

@@ -0,0 +1,12 @@
+using SVSim.Database.Models;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+public interface IArenaTwoPickCardPoolService
+{
+    /// <summary>
+    /// Returns exactly 2 candidate pairs for the requested turn. Ids assigned monotonically
+    /// (startingPairId, startingPairId+1); set_num = 1, 2; isSelected = false.
+    /// </summary>
+    List<CandidatePair> GeneratePickSetsForTurn(int classId, int turn, long startingPairId, IRandom rng);
+}