feat(engine): BestFitCache operates in canonical frame; TryPlaceBestFitPair builds from canonical drawing

OpenNest.Engine/BestFit/BestFitCache.cs

@@ -24,6 +24,9 @@ namespace OpenNest.Engine.BestFit
             if (_cache.TryGetValue(key, out var cached))
                 return cached;
 
+            // Operate on the canonical frame so cached pair positions are orientation-invariant.
+            var canonical = CanonicalFrame.AsCanonicalCopy(drawing);
+
             IPairEvaluator evaluator = null;
             ISlideComputer slideComputer = null;
 
@@ -31,7 +34,7 @@ namespace OpenNest.Engine.BestFit
             {
                 if (CreateEvaluator != null)
                 {
-                    try { evaluator = CreateEvaluator(drawing, spacing); }
+                    try { evaluator = CreateEvaluator(canonical, spacing); }
                     catch { /* fall back to default evaluator */ }
                 }
 
@@ -42,7 +45,7 @@ namespace OpenNest.Engine.BestFit
                 }
 
                 var finder = new BestFitFinder(plateWidth, plateHeight, evaluator, slideComputer);
-                var results = finder.FindBestFits(drawing, spacing, StepSize);
+                var results = finder.FindBestFits(canonical, spacing, StepSize);
 
                 _cache.TryAdd(key, results);
                 return results;
@@ -86,9 +89,12 @@ namespace OpenNest.Engine.BestFit
 
             try
             {
+                // Operate on the canonical frame so cached pair positions are orientation-invariant.
+                var canonical = CanonicalFrame.AsCanonicalCopy(drawing);
+
                 if (CreateEvaluator != null)
                 {
-                    try { evaluator = CreateEvaluator(drawing, spacing); }
+                    try { evaluator = CreateEvaluator(canonical, spacing); }
                     catch { /* fall back to default evaluator */ }
                 }
 
@@ -100,7 +106,7 @@ namespace OpenNest.Engine.BestFit
 
                 // Compute candidates and evaluate once with the largest plate.
                 var finder = new BestFitFinder(maxWidth, maxHeight, evaluator, slideComputer);
-                var baseResults = finder.FindBestFits(drawing, spacing, StepSize);
+                var baseResults = finder.FindBestFits(canonical, spacing, StepSize);
 
                 // Cache a filtered copy for each plate size.
                 foreach (var size in needed)

OpenNest.Engine/DefaultNestEngine.cs

@@ -139,6 +139,10 @@ namespace OpenNest
             var bestFits = BestFitCache.GetOrCompute(
                 drawing, Plate.Size.Length, Plate.Size.Width, Plate.PartSpacing);
 
+            // Build pair candidates with a canonical drawing so their geometry matches
+            // the coordinate frame of the cached fit results.
+            var canonicalDrawing = CanonicalFrame.AsCanonicalCopy(drawing);
+
             List<Part> bestPlacement = null;
 
             foreach (var fit in bestFits)
@@ -152,7 +156,7 @@ namespace OpenNest
                 if (fit.LongestSide > System.Math.Max(workArea.Width, workArea.Length) + Tolerance.Epsilon)
                     continue;
 
-                var landscape = fit.BuildParts(drawing);
+                var landscape = fit.BuildParts(canonicalDrawing);
                 var portrait = RotatePair90(landscape);
 
                 var lFits = TryOffsetToWorkArea(landscape, workArea);
@@ -174,6 +178,8 @@ namespace OpenNest
                     bestPlacement = candidate;
             }
 
+            // Parts are returned in canonical frame, bound to the canonical drawing.
+            // The outer Fill wrapper (Task 7) rebinds to `drawing` and composes sourceAngle onto rotation.
             return bestPlacement;
         }