feat(engine): expose NestJobCost, the benchmark's scoring

Engines optimized guesses at the benchmark cost: Opus55 re-implemented
salvage credit, Qwen used plate area per part area, Gpt6Astra ignored
salvage. NestJobCost moves StockLadder's EstimateNetArea into a public
home (net sheet area, unplaced-part penalty, whole-result Evaluate) and
the benchmark and StockLadder now call it. Scores are unchanged: tests pin
it against a frozen copy of the old computation and real benchmark runs.
Bounds still include marks, as before, so scores do not move.

Co-Authored-By: Codex <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Benchmark/BenchmarkRunner.cs

@@ -179,7 +179,7 @@ namespace OpenNest.Benchmark
                 var plateArea = plateRuns.Sum(run => run.Plate.Area());
                 var netSheetArea = validation.Valid
                     ? plateResults.Sum(result =>
-                        StockLadderNestingEngine.EstimateNetArea(baselineJob, result)
+                        NestJobCost.NetSheetArea(baselineJob, result)
                     )
                     : 0;
                 var sizeBreakdown = plateRuns
@@ -266,7 +266,7 @@ namespace OpenNest.Benchmark
                 // Salvage credit is recomputed from the job's own geometry, never taken from the engine.
                 var netSheetArea = validation.Valid
                     ? jobResult.Plates.Sum(p =>
-                        StockLadderNestingEngine.EstimateNetArea(nestJob, p)
+                        NestJobCost.NetSheetArea(nestJob, p)
                     )
                     : 0;
 

OpenNest.Benchmark/JobResult.cs

@@ -23,7 +23,7 @@ namespace OpenNest.Benchmark
         public double PlateArea { get; init; }
 
         /// <summary>Sheet area consumed after crediting salvageable offcuts
-        /// (StockLadderNestingEngine.EstimateNetArea summed over every plate).
+        /// (NestJobCost.NetSheetArea summed over every plate).
         /// Equals PlateArea when salvage credit is disabled.</summary>
         public double NetSheetArea { get; init; }