Slice 0 (Fable-Fixes): IClobClient-Seam fuer testbare CLOB-Interaktionen

Verhaltensneutrale Testinfrastruktur als Grundlage fuer Slice 1/2:
- IClobClient-Interface (Core) ueber die von Leiter/Reconciliation genutzten
  CLOB-Methoden (Place/CancelConflicting/GetOpenOrders/CancelOrder).
- PolymarketClobClient implementiert IClobClient (Signaturen unveraendert).
- DI-Seam im Modul: IClobClient -> PolymarketClobClient-Singleton.
- SellLadderService haengt jetzt an IClobClient (statische CalculateExactOrderAmounts
  bleibt am konkreten Typ).
- InternalsVisibleTo(PolyTrader.Tests) + FakeClobClient-Test-Double.

Verifikation: Build 0 Fehler, 207 Tests gruen, --smoke-ui Container-Aufbau ok.

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

src/PolyTrader.Modules.CopyTrading/Services/SellLadderService.cs

@@ -27,7 +27,7 @@ namespace PolyTraderSharp.Services
 
         private readonly CopyTradingState _copyState;
         private readonly TradingState _state;
-        private readonly PolymarketClobClient _clob;
+        private readonly IClobClient _clob;
         private readonly TerminalLogger _logger;
         private readonly ThreemaService _threema;
         private readonly IPositionRepository _positionRepo;
@@ -35,7 +35,7 @@ namespace PolyTraderSharp.Services
         public SellLadderService(
             CopyTradingState copyState,
             TradingState state,
-            PolymarketClobClient clob,
+            IClobClient clob,
             TerminalLogger logger,
             ThreemaService threema,
             IPositionRepository positionRepo)