feat: mirror axis simplifier, bend note propagation, ellipse fixes

Geometry Simplifier:
- Replace least-squares circle fitting with mirror axis algorithm
  that constrains center to perpendicular bisector of chord, guaranteeing
  zero-gap endpoint connectivity by construction
- Golden section search optimizes center position along the axis
- Increase default tolerance from 0.005 to 0.5 for practical CNC use
- Support existing arcs in simplification runs (sample arc points to
  find larger replacement arcs spanning lines + arcs together)
- Add tolerance zone visualization (offset original geometry ±tolerance)
- Show original geometry overlay with orange dashed lines in preview
- Add "Original" checkbox to CadConverter for comparing old vs new
- Store OriginalEntities on FileListItem to prevent tolerance creep
  when re-running simplifier with different settings

Bend Detection:
- Propagate bend notes to collinear bend lines split by cutouts
  using infinite-line perpendicular distance check
- Add bend note text rendering in EntityView at bend line midpoints

DXF Import:
- Fix trimmed ellipse closing chord: only close when sweep ≈ 2π,
  preventing phantom lines through slot cutouts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/Bending/SolidWorksBendDetectorTests.cs

@@ -29,6 +29,84 @@ public class SolidWorksBendDetectorTests
         Assert.Empty(bends);
     }
 
+    [Fact]
+    public void Simplifier_EllipseSegments_FewLargeArcs()
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Bending", "TestData", "4526 A14 PT11 Test.dxf");
+        Assert.True(File.Exists(path), $"Test DXF not found: {path}");
+
+        var importer = new OpenNest.IO.DxfImporter { SplinePrecision = 200 };
+        var result = importer.Import(path);
+
+        var shape = new OpenNest.Geometry.Shape();
+        shape.Entities.AddRange(result.Entities);
+
+        // Default tolerance is 0.5 — should produce very few large arcs
+        var simplifier = new OpenNest.Geometry.GeometrySimplifier();
+        var candidates = simplifier.Analyze(shape);
+
+        // With 0.5 tolerance, 2 ellipses (~400 segments) should reduce to a handful of arcs
+        // Dump for visibility then assert
+        var info = string.Join(", ", candidates.Select(c => $"[{c.StartIndex}..{c.EndIndex}]={c.LineCount}lines R={c.FittedArc.Radius:F3}"));
+        Assert.True(candidates.Count <= 10,
+            $"Expected <=10 arcs but got {candidates.Count}: {info}");
+
+        // Each arc should cover many lines
+        foreach (var c in candidates)
+            Assert.True(c.LineCount >= 3, $"Arc [{c.StartIndex}..{c.EndIndex}] only covers {c.LineCount} lines");
+
+        // Arcs should connect to the original geometry within tolerance
+        foreach (var c in candidates)
+        {
+            var firstLine = (OpenNest.Geometry.Line)shape.Entities[c.StartIndex];
+            var lastLine = (OpenNest.Geometry.Line)shape.Entities[c.EndIndex];
+            var arc = c.FittedArc;
+
+            var startGap = firstLine.StartPoint.DistanceTo(arc.StartPoint());
+            var endGap = lastLine.EndPoint.DistanceTo(arc.EndPoint());
+
+            Assert.True(startGap < 1e-9, $"Start gap {startGap} at candidate [{c.StartIndex}..{c.EndIndex}]");
+            Assert.True(endGap < 1e-9, $"End gap {endGap} at candidate [{c.StartIndex}..{c.EndIndex}]");
+        }
+    }
+
+    [Fact]
+    public void Import_TrimmedEllipse_NoClosingChord()
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Bending", "TestData", "4526 A14 PT11.dxf");
+        Assert.True(File.Exists(path), $"Test DXF not found: {path}");
+
+        var importer = new OpenNest.IO.DxfImporter();
+        var result = importer.Import(path);
+
+        // The DXF has 2 trimmed ellipses forming an oblong slot.
+        // Trimmed ellipses must not generate a closing chord line.
+        // 83 = 72 lines + 4 arcs + 7 circles + ellipse segments (heavily merged by optimizer)
+        Assert.Equal(83, result.Entities.Count);
+    }
+
+    [Fact]
+    public void DetectBends_SplitBendLine_PropagatesNote()
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Bending", "TestData", "4526 A14 PT23.dxf");
+        Assert.True(File.Exists(path), $"Test DXF not found: {path}");
+
+        using var reader = new DxfReader(path);
+        var doc = reader.Read();
+
+        var detector = new SolidWorksBendDetector();
+        var bends = detector.DetectBends(doc);
+
+        Assert.Equal(5, bends.Count);
+        Assert.All(bends, b =>
+        {
+            Assert.NotNull(b.NoteText);
+            Assert.Equal(BendDirection.Up, b.Direction);
+            Assert.Equal(90.0, b.Angle);
+            Assert.Equal(0.125, b.Radius);
+        });
+    }
+
     [Fact]
     public void DetectBends_RealDxf_ParsesNotesCorrectly()
     {