feat: add optional M98 part sub-programs to Cincinnati post processor

Each unique part geometry (drawing + rotation) is written once as a
reusable sub-program called via M98, reducing output size for nests
with repeated parts. G92 coordinate repositioning handles per-instance
plate placement with restore after each call. Cut-offs remain inline.

Controlled by UsePartSubprograms (default false) and PartSubprogramStart
config properties.

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

OpenNest.Posts.Cincinnati/CincinnatiPartSubprogramWriter.cs

@@ -0,0 +1,124 @@
+using System.Collections.Generic;
+using System.IO;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+
+namespace OpenNest.Posts.Cincinnati;
+
+/// <summary>
+/// Writes a Cincinnati-format part sub-program definition.
+/// Each sub-program contains the complete cutting sequence for one unique part geometry
+/// (drawing + rotation), with coordinates normalized to origin (0,0).
+/// Called via M98 from sheet sub-programs.
+/// </summary>
+public sealed class CincinnatiPartSubprogramWriter
+{
+    private readonly CincinnatiPostConfig _config;
+    private readonly CincinnatiFeatureWriter _featureWriter;
+
+    public CincinnatiPartSubprogramWriter(CincinnatiPostConfig config)
+    {
+        _config = config;
+        _featureWriter = new CincinnatiFeatureWriter(config);
+    }
+
+    /// <summary>
+    /// Writes a complete part sub-program for the given normalized program.
+    /// The program coordinates must already be normalized to origin (0,0).
+    /// </summary>
+    public void Write(TextWriter w, Program normalizedProgram, string drawingName,
+        int subNumber, string libraryFile, double sheetDiagonal)
+    {
+        var features = SplitFeatures(normalizedProgram.Codes);
+        if (features.Count == 0)
+            return;
+
+        w.WriteLine("(*****************************************************)");
+        w.WriteLine($":{subNumber}");
+        w.WriteLine(CoordinateFormatter.Comment($"PART: {drawingName}"));
+
+        for (var i = 0; i < features.Count; i++)
+        {
+            var codes = features[i];
+            var featureNumber = i == 0
+                ? _config.FeatureLineNumberStart
+                : 1000 + i + 1;
+            var cutDistance = ComputeCutDistance(codes);
+
+            var ctx = new FeatureContext
+            {
+                Codes = codes,
+                FeatureNumber = featureNumber,
+                PartName = drawingName,
+                IsFirstFeatureOfPart = false,
+                IsLastFeatureOnSheet = i == features.Count - 1,
+                IsSafetyHeadraise = false,
+                IsExteriorFeature = false,
+                LibraryFile = libraryFile,
+                CutDistance = cutDistance,
+                SheetDiagonal = sheetDiagonal
+            };
+
+            _featureWriter.Write(w, ctx);
+        }
+
+        w.WriteLine("G0X0Y0");
+        w.WriteLine($"M99(END OF {drawingName})");
+    }
+
+    /// <summary>
+    /// Creates a sub-program key for matching parts to their sub-programs.
+    /// </summary>
+    internal static (int drawingId, long rotationKey) SubprogramKey(Part part) =>
+        (part.BaseDrawing.Id, (long)System.Math.Round(part.Rotation * 1e6));
+
+    internal static List<List<ICode>> SplitFeatures(List<ICode> codes)
+    {
+        var features = new List<List<ICode>>();
+        List<ICode> current = null;
+
+        foreach (var code in codes)
+        {
+            if (code is RapidMove)
+            {
+                if (current != null)
+                    features.Add(current);
+                current = new List<ICode> { code };
+            }
+            else
+            {
+                current ??= new List<ICode>();
+                current.Add(code);
+            }
+        }
+
+        if (current != null && current.Count > 0)
+            features.Add(current);
+
+        return features;
+    }
+
+    internal static double ComputeCutDistance(List<ICode> codes)
+    {
+        var distance = 0.0;
+        var currentPos = Vector.Zero;
+
+        foreach (var code in codes)
+        {
+            if (code is RapidMove rapid)
+                currentPos = rapid.EndPoint;
+            else if (code is LinearMove linear)
+            {
+                distance += currentPos.DistanceTo(linear.EndPoint);
+                currentPos = linear.EndPoint;
+            }
+            else if (code is ArcMove arc)
+            {
+                distance += currentPos.DistanceTo(arc.EndPoint);
+                currentPos = arc.EndPoint;
+            }
+        }
+
+        return distance;
+    }
+}