perf(engine): key fill caches by source drawing so they hit across trials

Every DefaultPlateFiller.Fill makes a fresh canonical copy of the drawing,
and BestFitCache/FillResultCache keyed by drawing reference, so fills never
shared results and the static caches grew without bound.

CanonicalFrame now records which drawing each canonical copy came from.
Both caches key weakly on that source drawing, so every canonical copy
shares one entry and released drawings can be collected. An entry is
dropped when the drawing's Program instance or canonical angle changes.
Best-fit candidates are computed once per (drawing, spacing) through
BestFitFinder.FindCandidates and filtered per plate size with the same
filter FindBestFits uses. FillResultCache keeps canonical and
non-canonical callers apart.

Adds Debug-only PerfCounters for best-fit runs, offset perimeter builds
and Part.Intersects calls.

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

OpenNest.Engine/BestFit/BestFitFinder.cs

@@ -28,10 +28,19 @@ namespace OpenNest.Engine.BestFit
                 slideComputer != null
                     ? (IDistanceComputer)new GpuDistanceComputer(slideComputer)
                     : new CpuDistanceComputer();
+            _filter = CreateFilter(maxPlateWidth, maxPlateHeight);
+        }
+
+        /// <summary>
+        /// The filter <see cref="FindBestFits"/> applies for a plate of the given size. The
+        /// aspect-ratio limit widens with the plate's own aspect.
+        /// </summary>
+        public static BestFitFilter CreateFilter(double maxPlateWidth, double maxPlateHeight)
+        {
             var plateAspect =
                 System.Math.Max(maxPlateWidth, maxPlateHeight)
                 / System.Math.Max(System.Math.Min(maxPlateWidth, maxPlateHeight), 0.001);
-            _filter = new BestFitFilter
+            return new BestFitFilter
             {
                 MaxPlateWidth = maxPlateWidth,
                 MaxPlateHeight = maxPlateHeight,
@@ -46,6 +55,39 @@ namespace OpenNest.Engine.BestFit
             BestFitSortField sortBy = BestFitSortField.Area
         )
         {
+            var results = Evaluate(drawing, spacing, stepSize);
+
+            _filter.Apply(results);
+
+            return Number(SortResults(results, sortBy));
+        }
+
+        /// <summary>
+        /// Evaluates every pair candidate without the plate-size filter, sorted by area. Applying
+        /// <see cref="CreateFilter"/> for a plate size to shallow copies of these results gives
+        /// the same results <see cref="FindBestFits"/> returns for that size, so one run can
+        /// serve several sizes.
+        /// </summary>
+        public List<BestFitResult> FindCandidates(
+            Drawing drawing,
+            double spacing = 0.25,
+            double stepSize = 0.25
+        )
+        {
+            return Number(SortResults(Evaluate(drawing, spacing, stepSize), BestFitSortField.Area));
+        }
+
+        private static List<BestFitResult> Number(List<BestFitResult> results)
+        {
+            for (var i = 0; i < results.Count; i++)
+                results[i].Candidate.TestNumber = i;
+
+            return results;
+        }
+
+        private List<BestFitResult> Evaluate(Drawing drawing, double spacing, double stepSize)
+        {
+            PerfCounters.CountFindBestFits();
             var strategies = BuildStrategies(drawing, spacing);
 
             var candidateBags = new ConcurrentBag<List<PairCandidate>>();
@@ -64,16 +106,7 @@ namespace OpenNest.Engine.BestFit
                 $"[BestFitFinder] {strategies.Count} strategies, {allCandidates.Count} candidates"
             );
 
-            var results = _evaluator.EvaluateAll(allCandidates);
-
-            _filter.Apply(results);
-
-            results = SortResults(results, sortBy);
-
-            for (var i = 0; i < results.Count; i++)
-                results[i].Candidate.TestNumber = i;
-
-            return results;
+            return _evaluator.EvaluateAll(allCandidates);
         }
 
         public List<TileResult> FindAndTile(