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/JobLoader.cs

@@ -81,6 +81,29 @@ namespace OpenNest.Benchmark
             return jobs;
         }
 
+        /// <summary>Parses "WxL" with invariant-culture numbers, so "48.5x96" means the
+        /// same thing on every machine (Size.Parse follows the current culture).</summary>
+        public static bool TryParseSheetSize(string text, out Size size)
+        {
+            size = default;
+            var dims = text?.Split('x', 'X');
+
+            if (dims == null || dims.Length != 2)
+                return false;
+
+            var style = System.Globalization.NumberStyles.Float;
+            var culture = System.Globalization.CultureInfo.InvariantCulture;
+
+            if (
+                !double.TryParse(dims[0].Trim(), style, culture, out var width)
+                || !double.TryParse(dims[1].Trim(), style, culture, out var length)
+            )
+                return false;
+
+            size = new Size(width, length);
+            return true;
+        }
+
         private static List<string> ResolveFiles(string inputPath)
         {
             if (Directory.Exists(inputPath))