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/MarketData.cs

@@ -0,0 +1,21 @@
+namespace PolyTraderSharp.Models
+{
+    public class MarketData
+    {
+        [MongoDB.Bson.Serialization.Attributes.BsonId] public string Id { get; set; } = string.Empty;
+        public string ConditionId { get; set; } = string.Empty;
+        public string Question { get; set; } = string.Empty;
+        public string Slug { get; set; } = string.Empty;
+        public DateTime? EndDate { get; set; }
+        public bool Active { get; set; }
+        public bool Closed { get; set; }
+        public string Category { get; set; } = string.Empty;
+
+        // Will store the JSON array string of token IDs, e.g. "[\"123\", \"456\"]"
+        public string ClobTokenIds { get; set; } = string.Empty;
+        
+        public string Outcomes { get; set; } = string.Empty; // e.g. "[\"Yes\", \"No\"]"
+        
+        public bool NegRisk { get; set; }
+    }
+}