fix(benchmark): rank by sheet cost so engines can't game the score

The benchmark is about to be used as the objective for LLM-designed
engines, and several gaps would have rewarded the wrong behavior:

- Ranking was utilization-first, so dropping awkward parts raised the
  score. Rank valid > fully placed > cost > plates, where cost is
  salvage-credited sheet area plus a largest-sheet penalty per unplaced
  part; placing a part is never scored worse than omitting it.
- Salvage rate was ignored in scoring; cost now uses EstimateNetArea,
  recomputed from job geometry rather than trusted from the engine.
- Rotation constraints were never validated. Add RotationPolicy.Allows
  (shared with NestJobPlacementValidator) and check every placement.
- Returned sheets were trusted, so an engine could loosen spacing or
  invent a size. Sheets must now match offered stock.
- Part-in-part placements were flagged as overlaps; spacing now accounts
  for cutouts, with an X-sorted sweep to prune distant pairs.
- Summary averaged per-job percentages; it now sums areas and cost.
- --spacing and sheet sizes parsed with the current culture.
- Warn when .nest jobs offer only their original sheet sizes.

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

OpenNest.Benchmark/BenchmarkJob.cs

@@ -42,6 +42,12 @@ namespace OpenNest.Benchmark
 
         public int TotalRequestedQuantity => Requests.Sum(r => r.Quantity);
 
+        /// <summary>Sheet area charged per unplaced part: the largest candidate
+        /// sheet. Any single part that fits the stock at all fits on one such
+        /// sheet, so placing a part is never scored worse than leaving it out.</summary>
+        public double UnplacedPartPenalty =>
+            CandidateSizes.Count == 0 ? 0 : CandidateSizes.Max(s => s.Width * s.Length);
+
         /// <summary>
         /// Builds the whole-job request this job represents: one NestJobPart per
         /// requested drawing, and one NestPlateStock per candidate sheet size