fix: Cincinnati post processor arc feedrate, G89 spacing, pallet exchange, and preamble

- Add radius-based arc feedrate calculation (Variables/Percentages modes)
  with configurable radius ranges (#123/#124/#125 or inline expressions)
- Fix arc distance in SpeedClassifier using actual arc length instead of
  chord length (full circles previously computed as zero)
- Fix G89 P spacing: P now adjacent to filename per CL-707 manual syntax
- Add lead-out feedrate support (#129) and arc lead-in feedrate (#127)
- Fix pallet exchange: StartAndEnd emits M50 in preamble + last sheet only
- Add G121 Smart Rapids emission when UseSmartRapids is enabled
- Add G90 absolute mode to main program preamble alongside G20/G21

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

OpenNest.Posts.Cincinnati/CincinnatiPreambleWriter.cs

@@ -37,15 +37,21 @@ public sealed class CincinnatiPreambleWriter
 
         w.WriteLine(CoordinateFormatter.Comment("MAIN PROGRAM"));
 
-        w.WriteLine(_config.PostedUnits == Units.Millimeters ? "G21" : "G20");
+        w.WriteLine(_config.PostedUnits == Units.Millimeters ? "G21 G90" : "G20 G90");
+
+        if (_config.UseSmartRapids)
+            w.WriteLine("G121 (SMART RAPIDS)");
 
         w.WriteLine("M42");
 
         if (_config.ProcessParameterMode == G89Mode.LibraryFile && !string.IsNullOrEmpty(initialLibrary))
-            w.WriteLine($"G89 P {initialLibrary}");
+            w.WriteLine($"G89 P{initialLibrary}");
 
         w.WriteLine($"M98 P{_config.VariableDeclarationSubprogram} (Variable Declaration)");
 
+        if (_config.PalletExchange == PalletMode.StartAndEnd)
+            w.WriteLine("M50");
+
         w.WriteLine("GOTO1 (GOTO SHEET NUMBER)");
 
         for (var i = 1; i <= sheetCount; i++)