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.Core/BestFit/IBestFitStore.cs

@@ -21,12 +21,17 @@ namespace OpenNest.BestFit
         IReadOnlyList<double> HullAngles
     );
 
-    /// <summary>The saved best-fit results for one drawing at one plate size and spacing.</summary>
+    /// <summary>
+    /// The saved best-fit results for one drawing at one plate size and spacing.
+    /// <paramref name="CanonicalAngle"/> is the drawing's canonical angle (radians) when the
+    /// set was saved: the frame its pair geometry is in. Null means the frame is unknown.
+    /// </summary>
     public sealed record SavedPairSet(
         double PlateWidth,
         double PlateHeight,
         double Spacing,
-        IReadOnlyList<SavedPairResult> Results
+        IReadOnlyList<SavedPairResult> Results,
+        double? CanonicalAngle = null
     );
 
     /// <summary>
@@ -45,7 +50,10 @@ namespace OpenNest.BestFit
             IReadOnlySet<(double Width, double Length, double Spacing)> plateKeys
         );
 
-        /// <summary>Restores saved sets for <paramref name="drawing"/>.</summary>
+        /// <summary>
+        /// Restores saved sets for <paramref name="drawing"/>. Sets saved in a frame other
+        /// than the drawing's current canonical frame, or in an unknown one, are stale.
+        /// </summary>
         void Import(Drawing drawing, IReadOnlyList<SavedPairSet> sets);
     }
 }