Phase 3a: Core-Domänenmodelle in den Core verschoben

- AccountState, Position, MarketData nach src/PolyTrader.Core/Models/
  (Namespace PolyTraderSharp.Models beibehalten → keine Consumer-Änderung).
- AccountState: ungenutzte usings (MongoDB.Driver, PolyTraderSharp.Extensions)
  entfernt (Core darf App-Shim nicht referenzieren).
- MongoDB.Driver 2.24.0 als Core-Paket (für BsonId-Attribute).
- Build 0 Fehler.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PolyTrader.Core/Models/Position.cs

@@ -0,0 +1,23 @@
+namespace PolyTraderSharp.Models
+{
+    public class Position
+    {
+        [MongoDB.Bson.Serialization.Attributes.BsonId]
+        public string TokenId { get; set; } = string.Empty;
+        public string MarketSlug { get; set; } = string.Empty;
+        public string ConditionId { get; set; } = string.Empty;
+        public int SourceTraderId { get; set; }
+        public string SourceTraderName { get; set; } = string.Empty;
+        public string SourceTraderAddress { get; set; } = string.Empty;
+        public string MarketQuestion { get; set; } = string.Empty;
+        public string Outcome { get; set; } = string.Empty;
+        public string Side { get; set; } = "BUY";
+        public decimal EntryPrice { get; set; }
+        public decimal Size { get; set; } // Shares
+        public decimal AmountUsd { get; set; }
+        public decimal CurrentPrice { get; set; }
+        public decimal CurrentValueUsd { get; set; }
+        public System.DateTime? ExpiryDate { get; set; }
+        public System.DateTime OpenedAt { get; set; } = System.DateTime.UtcNow;
+    }
+}