fix(geometry): treat near-joint contacts as the joint when classifying slides

Drawing data can fix a fillet's center slightly off the tangent normal, so
the arc's end sits ~1e-7 past its neighbouring line's end. The slide contact
classifier only joined entities within 1e-7, so a contact at that tangent
point read as two boundaries, was classified ambiguous and blocked the push.
In N26-243 this stopped a part pushed left along another once the parts
aligned. Contacts within 1e-6 of a joint now take the joint's tangents;
the 1e-7 incidence tolerance is unchanged.

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

OpenNest.Core/Geometry/SlideContact.cs

@@ -36,6 +36,11 @@ namespace OpenNest.Geometry
         // tangential classification can admit, so keep it far below spacing tolerances.
         internal const double IncidenceTolerance = 1e-7;
 
+        // Drawing data fixes arc centers to finite precision, so a tangent joint's arc end
+        // can sit a little past its neighbour's end. A contact this close to a joint takes
+        // the joint's tangents; otherwise the neighbour reads as a second boundary there.
+        internal const double JointTolerance = 1e-6;
+
         private readonly List<Entity> entities;
         private readonly int[] loopOf;
         private readonly int[] previous;
@@ -254,11 +259,11 @@ namespace OpenNest.Geometry
 
         private bool StartsNear(int index, Vector point) =>
             TryEndpoints(entities[index], out var start, out _)
-            && start.DistanceTo(point) <= IncidenceTolerance;
+            && start.DistanceTo(point) <= JointTolerance;
 
         private bool EndsNear(int index, Vector point) =>
             TryEndpoints(entities[index], out _, out var end)
-            && end.DistanceTo(point) <= IncidenceTolerance;
+            && end.DistanceTo(point) <= JointTolerance;
 
         private bool InRun(int index, int first, int last)
         {

OpenNest.Tests/Geometry/SlideContactTests.cs

@@ -147,6 +147,28 @@ public class SlideContactTests
         Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(1, 0)));
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void FilletWithImpreciseCenter_DoesNotBlockSlidingAlongItsTangentLine(bool cpu)
+    {
+        // N26-243: the G-code fixes the fillet center 1.04e-7 off the tangent normal, so
+        // the arc's top lies just past the flat edge's end rather than on the joint.
+        var offset = 1.043e-7;
+        var fillet = new Arc(new Vector(2 - offset, 0.5), System.Math.Sqrt(1 + offset * offset),
+            0, System.Math.Atan2(1, offset), false);
+        var stationary = new List<Entity>
+        {
+            new Line(0, 0, 3, 0),
+            new Line(new Vector(3, 0), fillet.StartPoint()),
+            fillet,
+            new Line(fillet.EndPoint(), new Vector(0, 1.5)),
+            new Line(0, 1.5, 0, 0),
+        };
+        var moving = Rect(1, 1.5, 2, 1).Cast<Entity>().ToList();
+        Assert.Equal(double.MaxValue, EntityDistance(cpu, moving, stationary, new Vector(-1, 0)));
+    }
+
     [Theory]
     [InlineData(false)]
     [InlineData(true)]