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/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; }