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.IO/Extensions.cs

@@ -221,8 +221,9 @@ namespace OpenNest.IO
                 });
             }
 
-            // Close the ellipse if it's a full ellipse
-            if (lines.Count >= 2)
+            // Close only if it's a full ellipse (sweep ≈ 2π)
+            var sweep = endParam - startParam;
+            if (lines.Count >= 2 && System.Math.Abs(sweep - System.Math.PI * 2.0) < 0.01)
             {
                 var first = lines.First();
                 var last = lines.Last();