fix(bestfit): drop saved pair sets whose canonical frame changed

Saved best-fit pairs are in the drawing's canonical frame, but the canonical
angle is recomputed on load and was not saved with the set. When that
computation changes (RotatingCalipers checking every hull edge, marks no
longer counted as material), old pairs were replayed in a turned frame and
could overlap.

Each saved set now records canonicalAngle, and BestFitCacheStore.Import
skips sets whose angle no longer matches the recomputed one (1e-9 rad,
modulo a full turn). Sets from older files have no angle and are dropped;
their pairs are recomputed when next needed.

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

OpenNest.Engine/BestFit/BestFitCacheStore.cs

@@ -8,9 +8,17 @@ namespace OpenNest.Engine.BestFit
     /// <summary>
     /// Saves and restores <see cref="BestFitCache"/> entries for .nest archives. Only kept
     /// results are exported; imported sets are added without replacing computed ones.
+    /// Each set records the canonical angle its pairs were computed in, and import drops sets
+    /// whose angle no longer matches the drawing's recomputed one.
     /// </summary>
     public sealed class BestFitCacheStore : IBestFitStore
     {
+        /// <summary>
+        /// Largest difference (radians) at which a saved canonical angle still matches. The
+        /// angle round-trips exactly, so this only absorbs floating-point noise in recomputing it.
+        /// </summary>
+        private const double FrameTolerance = 1e-9;
+
         /// <summary>The store over the process-wide <see cref="BestFitCache"/>.</summary>
         public static BestFitCacheStore Shared { get; } = new();
 
@@ -50,7 +58,8 @@ namespace OpenNest.Engine.BestFit
                     ))
                     .ToList();
 
-                sets.Add(new SavedPairSet(kvp.Key.PlateWidth, kvp.Key.PlateHeight, kvp.Key.Spacing, results));
+                sets.Add(new SavedPairSet(
+                    kvp.Key.PlateWidth, kvp.Key.PlateHeight, kvp.Key.Spacing, results, FrameAngle(drawing)));
             }
 
             return sets;
@@ -64,7 +73,7 @@ namespace OpenNest.Engine.BestFit
 
             foreach (var set in sets)
             {
-                if (set.Results.Count == 0)
+                if (set.Results.Count == 0 || !MatchesFrame(set, drawing))
                     continue;
 
                 canonical ??= CanonicalFrame.AsCanonicalCopy(drawing);
@@ -95,5 +104,21 @@ namespace OpenNest.Engine.BestFit
                 BestFitCache.Populate(drawing, set.PlateWidth, set.PlateHeight, set.Spacing, results);
             }
         }
+
+        private static double FrameAngle(Drawing drawing) => drawing.Source?.Angle ?? 0.0;
+
+        /// <summary>
+        /// True when the set was saved in the drawing's current canonical frame. A changed
+        /// canonical-angle computation would replay old pairs in a turned frame, where they can
+        /// overlap; sets with no saved angle (older files) are treated the same way.
+        /// </summary>
+        private static bool MatchesFrame(SavedPairSet set, Drawing drawing)
+        {
+            if (set.CanonicalAngle is not double saved || !double.IsFinite(saved))
+                return false;
+
+            var difference = System.Math.IEEERemainder(saved - FrameAngle(drawing), 2.0 * System.Math.PI);
+            return System.Math.Abs(difference) <= FrameTolerance;
+        }
     }
 }