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.Engine/Jobs/NestJobPlacementValidator.cs

@@ -43,7 +43,7 @@ internal static class NestJobPlacementValidator
             counts.TryGetValue(placement.PartId, out var count);
             if (count >= available)
                 throw new InvalidOperationException("Candidate overproduces a requirement.");
-            if (!RotationIsAllowed(part.Rotation, placement.Rotation))
+            if (!part.Rotation.Allows(placement.Rotation))
                 throw new InvalidOperationException(
                     "Candidate rotation is not allowed for the requirement."
                 );
@@ -78,25 +78,6 @@ internal static class NestJobPlacementValidator
         _ = CreateShape(geometry);
     }
 
-    private static bool RotationIsAllowed(RotationPolicy policy, double rotation)
-    {
-        if (policy.Kind == RotationPolicyKind.Automatic)
-            return true;
-        if (policy.Kind == RotationPolicyKind.Fixed)
-            return AnglesEqual(rotation, policy.Start);
-        if (rotation < policy.Start - Epsilon || rotation > policy.End + Epsilon)
-            return false;
-        var steps = (rotation - policy.Start) / policy.Step;
-        return System.Math.Abs(steps - System.Math.Round(steps)) <= Epsilon;
-    }
-
-    private static bool AnglesEqual(double left, double right)
-    {
-        var delta = (left - right) % (System.Math.PI * 2);
-        return System.Math.Abs(delta) <= Epsilon
-            || System.Math.Abs(System.Math.Abs(delta) - System.Math.PI * 2) <= Epsilon;
-    }
-
     private static ShapeTopology CreateShape(PartGeometrySnapshot geometry)
     {
         var entities = ConvertProgram.ToGeometry(DrawingJobMapper.ToProgram(geometry));