refactor(posts): move post-processor projects under Posts/

Group the Cincinnati and GravographIS plugin projects in a Posts/
folder so new machine posts have one home. Project names, namespaces,
and the runtime Posts/ deploy target are unchanged; only relative
paths in the solution and project references move.

Posts/OpenNest.Posts.Cincinnati/MaterialLibraryNameConverter.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+
+namespace OpenNest.Posts.Cincinnati
+{
+    public sealed class MaterialLibraryNameConverter : StringConverter
+    {
+        public override bool GetStandardValuesSupported(ITypeDescriptorContext context) => true;
+
+        public override bool GetStandardValuesExclusive(ITypeDescriptorContext context) => false;
+
+        public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
+        {
+            var config = context?.Instance as CincinnatiPostConfig;
+            var names = new List<string> { "" };
+
+            if (config?.MaterialLibraries != null)
+            {
+                names.AddRange(
+                    config
+                        .MaterialLibraries.Select(e => e.Library)
+                        .Where(s => !string.IsNullOrWhiteSpace(s))
+                        .Distinct(StringComparer.OrdinalIgnoreCase)
+                        .OrderBy(s => s, StringComparer.OrdinalIgnoreCase)
+                );
+            }
+
+            return new StandardValuesCollection(names);
+        }
+    }
+}