fix: show both offset and rotation in SubProgramCall.ToString

The either/or format meant a SubProgramCall with both a non-zero
Offset and non-zero Rotation would only show the Offset, hiding the
rotation metadata. The data model supports both independently, so the
display should too.

Also fixes a zero-field leak where the old fallback emitted
`G65 P_ R0` for calls with no rotation. Now each field is only shown
when non-zero, and `G65 P_` with no arguments is emitted when
neither is set.

Note: SubProgramCall.ToString is purely a debug/display aid. The
Cincinnati post emits sub-calls via the G52 + M98 bracket, not via
G65, so this format doesn't correspond to real machine output.

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

OpenNest.Core/CNC/SubProgramCall.cs

@@ -1,4 +1,5 @@
-using OpenNest.Geometry;
+using System.Text;
+using OpenNest.Geometry;
 using OpenNest.Math;
 
 namespace OpenNest.CNC
@@ -90,9 +91,13 @@ namespace OpenNest.CNC
 
         public override string ToString()
         {
+            var sb = new StringBuilder();
+            sb.Append($"G65 P{Id}");
             if (Offset.X != 0 || Offset.Y != 0)
-                return string.Format("G65 P{0} X{1} Y{2}", Id, Offset.X, Offset.Y);
-            return string.Format("G65 P{0} R{1}", Id, Rotation);
+                sb.Append($" X{Offset.X} Y{Offset.Y}");
+            if (Rotation != 0)
+                sb.Append($" R{Rotation}");
+            return sb.ToString();
         }
     }
 }

OpenNest.Tests/CuttingStrategy/HoleSubProgramTests.cs

@@ -44,6 +44,25 @@ public class HoleSubProgramTests
         Assert.Contains("Y2.5", str);
     }
 
+    [Fact]
+    public void SubProgramCall_ToString_IncludesOffsetAndRotation()
+    {
+        var call = new SubProgramCall { Id = 1000, Offset = new Vector(1.5, 2.5), Rotation = 30 };
+        var str = call.ToString();
+        Assert.Contains("P1000", str);
+        Assert.Contains("X1.5", str);
+        Assert.Contains("Y2.5", str);
+        Assert.Contains("R30", str);
+    }
+
+    [Fact]
+    public void SubProgramCall_ToString_OmitsZeroFields()
+    {
+        var call = new SubProgramCall { Id = 1000 };
+        var str = call.ToString();
+        Assert.Equal("G65 P1000", str);
+    }
+
     [Fact]
     public void Program_SubPrograms_EmptyByDefault()
     {