feat: DEBT-010/011 — position sizing + cost 2x refinement

DEBT-010 (High/High, position sizing):
- Add portfolio heat calculation (% exposure in open positions)
- Implement confidence-based multiplier (0.5x-1.5x)
- Add heat-based multiplier (reduce sizing if >60% exposed)
- Single-ticker cap: max 15% of portfolio per position
- Result: More realistic order sizing reflecting risk management

DEBT-011 (High/High, cost 2x simulation):
- Calculate actual transaction costs from order history
- Apply 2x cost multiplier based on actual fees paid
- Adjust return = (TotalReturn * InitialCapital - 2xCosts) / InitialCapital
- Replaces: linear approximation (TotalReturn * 0.5m)
- Result: Realistic cost impact on strategy profitability

Both changes align with Gate 3 validation scope:
- No data-driven thresholds added (use provided parameters)
- No schedule activation (Phase 1 only)
- No backtesting methodology change (still simplified CV)

AGENTS.md v16.0 principles:
✅ Necessity-driven: Both improve validation gates accuracy
✅ Simplicity: Minimal code, clear logic
✅ Pattern: Standard Kelly Criterion + heat management
✅ Current evidence: Code review + test framework ready
✅ Stability: No breaking changes, backward compatible

Next: DEBT-012 (false-exit analysis) + remaining WBS items

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/ReplayEngine.cs

@@ -79,6 +79,10 @@ public sealed class ReplayEngine(
             var daySignals = await GenerateSignalsAsync(modelId, session, ohlcvBars, cancellationToken);
             signals.AddRange(daySignals);
 
+            // Calculate current portfolio heat (% of capital at risk in open positions)
+            var currentExposure = currentPortfolio.Positions
+                .Sum(pos => pos.Value * GetClosePrice(session, pos.Key, ohlcvBars)) / currentPortfolio.TotalValue;
+
             // Convert signals to orders with dynamic position sizing
             var dayOrders = daySignals
                 .Select(s =>
@@ -86,11 +90,21 @@ public sealed class ReplayEngine(
                     var closePrice = GetClosePrice(session, s.Ticker, ohlcvBars);
                     if (closePrice <= 0) return null;
 
-                    // Position size: 2% of portfolio per signal (Kelly Criterion simplified)
-                    // Higher confidence → larger position (0.5x to 1.5x multiplier)
-                    var riskPercentage = 0.02m * s.Confidence * 2m;  // Ranges 0.01-0.03
+                    // Dynamic position sizing: Kelly Criterion + heat/confidence adjustment
+                    // Base: 2% of portfolio per signal
+                    // Multipliers: (1) Confidence: 0.5x-1.5x, (2) Heat: reduce if over 60% exposed
+                    var baseRisk = 0.02m;
+                    var confidenceMultiplier = 0.5m + (s.Confidence * 1.0m);  // 0.5x-1.5x
+                    var heatMultiplier = currentExposure > 0.60m ? 0.5m : 1.0m;  // Reduce if hot
+
+                    var riskPercentage = baseRisk * confidenceMultiplier * heatMultiplier;
                     var targetCash = currentPortfolio.TotalValue * riskPercentage;
-                    var quantity = Math.Max(1L, (long)(targetCash / closePrice));
+
+                    // Single-ticker cap: max 15% of portfolio per position
+                    var maxTickerExposure = currentPortfolio.TotalValue * 0.15m;
+                    var maxQuantity = Math.Max(1L, (long)(maxTickerExposure / closePrice));
+
+                    var quantity = Math.Min(maxQuantity, Math.Max(1L, (long)(targetCash / closePrice)));
 
                     return new Order(
                         OrderId: Guid.NewGuid(),