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/Line.cs

@@ -398,11 +398,9 @@ namespace OpenNest.Geometry
             var x = System.Math.Cos(angle) * distance;
             var y = System.Math.Sin(angle) * distance;
 
-            var pt = new Vector(x, y);
+            var pt = side == OffsetSide.Left ? new Vector(x, y) : new Vector(-x, -y);
 
-            return side == OffsetSide.Left
-                ? new Line(StartPoint + pt, EndPoint + pt)
-                : new Line(EndPoint + pt, StartPoint + pt);
+            return new Line(StartPoint + pt, EndPoint + pt);
         }
 
         public override Entity OffsetEntity(double distance, Vector pt)