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/GeometrySimplifierTests.cs

@@ -77,9 +77,10 @@ public class GeometrySimplifierTests
     }
 
     [Fact]
-    public void Analyze_MixedEntitiesWithArc_OnlyAnalyzesLines()
+    public void Analyze_MixedEntitiesWithArc_FindsSeparateCandidates()
     {
-        // Line, Line, Line, Arc, Line, Line, Line — should find candidates only in line runs
+        // Lines on one curve, then an arc at a different center, then lines on another curve
+        // The arc is included in the run but can't merge with lines on different curves
         var shape = new Shape();
         // First run: 5 lines on a curve
         var arc1 = new Arc(new Vector(0, 0), 10, 0, System.Math.PI / 2, false);