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.Host/Jobs/ShadowRunJob.cs

@@ -134,10 +134,16 @@ public sealed class ShadowRunJob(
 
             LogPhase4Complete(logger, command.RunId, null);
 
+            // Cost 2x scenario: simulate with double transaction fees
+            var actualTotalCost = CalculateTotalCostsFromOrders(replayResult.Orders, feeSchedule, ohlcvBars);
+            var twoXFeesCost = actualTotalCost * 2m;  // Double the actual transaction costs paid
+            var initialPortfolioValue = 10_000_000m;  // Match ReplayEngine initialization
+            var twoXCostReturn = (metrics.TotalReturn * initialPortfolioValue - twoXFeesCost) / initialPortfolioValue;
+
             var costAnalysis = new CostAnalysis(
                 BaseScenarioReturn: metrics.TotalReturn,
-                TwoXCostReturn: metrics.TotalReturn * 0.5m,  // Simplified: linear cost impact
-                PassesTwoXPositive: metrics.TotalReturn * 0.5m > 0);
+                TwoXCostReturn: twoXCostReturn,  // Actual 2x fee impact
+                PassesTwoXPositive: twoXCostReturn > 0);
 
             var falseExitAnalysis = new FalseExitAnalysis(
                 FalseExitCount: 0,              // TODO: Computed from signals
@@ -252,4 +258,25 @@ public sealed class ShadowRunJob(
             Sharpe: dto.Sharpe,
             WinRate: dto.WinRate,
             MaxDrawdown: dto.MaxDrawdown);
+
+    private static decimal CalculateTotalCostsFromOrders(
+        IReadOnlyList<ReplayEngine.Order> orders,
+        IReadOnlyList<DataBackfiller.FeeScheduleEntry> feeSchedule,
+        IReadOnlyList<DataBackfiller.OhlcvBar> ohlcvBars)
+    {
+        decimal totalCosts = 0m;
+
+        foreach (var order in orders.Where(o => o.FilledPrice.HasValue))
+        {
+            var cost = order.Quantity * order.FilledPrice.Value;
+
+            // Get fee schedule for this order's date
+            var fee = feeSchedule.FirstOrDefault(f => f.EffectiveDate <= order.FilledDate);
+            var feePercent = fee?.TransactionFeePercent ?? 0.001m;
+
+            totalCosts += cost * feePercent;
+        }
+
+        return totalCosts;
+    }
 }