fix(geometry): harden the arc-preserving per-entity offset

GetOffsetPerimeterEntities/GetOffsetPartEntities feed directional-distance
loops (FillLinear, Compactor, RotationSlideStrategy) that handle arcs
natively. Switching them to Clipper line output (plan option B) made
OpenNest.Tests run 48s -> 8m19s, Fill tests ~3x slower, and broke 20
exact-fit tests through tessellation and conservative padding, so they keep
the per-entity offset (option A), hardened:

- Arc, Circle and Line offsets are now side-symmetric. Right on a CCW arc
  shrank instead of growing, Right on a CW circle grew, and Right on a line
  offset to the left and reversed it. Only Left was used on hot paths, so
  this was latent (SimplifierViewer drew both tolerance bands on one side).
- Shape.OffsetEntity closes every gap between consecutive offset pieces:
  convex non-tangent line/arc corners get a round join about the original
  corner, lines across a collapsed fillet are mitered, and any other gap
  (concave arc corner, collapsed entity) is bridged with a line. Before,
  only line-line corners were joined, so a vertex could slip through.
- Zero-area spikes are left in place and documented: they lie inside the
  offset envelope, which is harmless for directional distance.
- OffsetOutward/OffsetInward become internal; PartGeometry is their only
  caller.

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

OpenNest.Core/Geometry/Arc.cs

@@ -443,19 +443,23 @@ namespace OpenNest.Geometry
             boundingBox.Width = maxY - minY;
         }
 
+        /// <summary>
+        /// Offsets the arc to the given side of its travel direction. The center lies to
+        /// the left of a CCW arc and to the right of a CW (reversed) one, so the arc grows
+        /// on the other side and shrinks toward its center. Returns null when it shrinks
+        /// to nothing.
+        /// </summary>
         public override Entity OffsetEntity(double distance, OffsetSide side)
         {
-            if (side == OffsetSide.Left && reversed)
-            {
-                return new Arc(center, radius + distance, startAngle, endAngle, reversed);
-            }
-            else
-            {
-                if (distance >= radius)
-                    return null;
+            var grows = (side == OffsetSide.Left) == reversed;
 
-                return new Arc(center, radius - distance, startAngle, endAngle, reversed);
-            }
+            if (grows)
+                return new Arc(center, radius + distance, startAngle, endAngle, reversed);
+
+            if (distance >= radius)
+                return null;
+
+            return new Arc(center, radius - distance, startAngle, endAngle, reversed);
         }
 
         public override Entity OffsetEntity(double distance, Vector pt)