feat: handle SubProgramCalls in Cincinnati post feature splitting

SubProgramCalls are now treated as standalone features in the Cincinnati
post-processor. SplitByRapids emits them as single-element features
instead of splitting on rapids within sub-programs. A nest-level hole
sub-program registry deduplicates by content and assigns post numbers.
Sheet writers emit M98 calls with X/Y offsets for hole features, and
hole sub-program definitions are written after part sub-programs.

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

OpenNest.Posts.Cincinnati/FeatureUtils.cs

@@ -21,7 +21,16 @@ public static class FeatureUtils
 
         foreach (var code in codes)
         {
-            if (code is RapidMove)
+            if (code is SubProgramCall)
+            {
+                // Flush any pending feature
+                if (current != null)
+                    features.Add(current);
+                // SubProgramCall is its own feature
+                features.Add(new List<ICode> { code });
+                current = null;
+            }
+            else if (code is RapidMove)
             {
                 if (current != null)
                     features.Add(current);