ENH: Use lossless float serialization in SpatialObject and Mesh IO

Two persistence-layer paths used lossy default stream precision for
floating-point coordinates:

itkPolygonGroupSpatialObjectXMLFile.cxx: PolygonSpatialObject vertex
coordinates (Point<double,3>) were streamed to the XML file with default
6-digit precision.  Replace with itk::ConvertNumberToString() per component.

itkFreeSurferAsciiMeshIO.h WritePoints<T>: hardcoded precision(6) with
std::fixed limited output to 6 decimal places regardless of T.  Replace
with itk::ConvertNumberToString() for both float and double specializations,
producing the shortest decimal string that round-trips exactly.

See: #3249

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hjmjohnson

Modules/IO/MeshFreeSurfer/include/itkFreeSurferAsciiMeshIO.h

@@ -24,6 +24,7 @@
 #include "itkMakeUniqueForOverwrite.h"
 
 #include <fstream>
+#include "itkNumberToString.h"
 
 namespace itk
 {
@@ -117,13 +118,12 @@ class ITKIOMeshFreeSurfer_EXPORT FreeSurferAsciiMeshIO : public MeshIOBase
   void
   WritePoints(T * buffer, std::ofstream & outputFile, T label = T{})
   {
-    outputFile.precision(6);
     SizeValueType index = 0;
     for (SizeValueType ii = 0; ii < this->m_NumberOfPoints; ++ii)
     {
       for (unsigned int jj = 0; jj < this->m_PointDimension; ++jj)
       {
-        outputFile << std::fixed << buffer[index++] << "  ";
+        outputFile << itk::ConvertNumberToString(buffer[index++]) << "  ";
       }
       outputFile << label << '\n';
     }

Modules/IO/SpatialObjects/src/itkPolygonGroupSpatialObjectXMLFile.cxx

@@ -20,6 +20,7 @@
 #include "itksys/SystemTools.hxx"
 #include "itkMetaDataObject.h"
 #include "itkIOCommon.h"
+#include "itkNumberToString.h"
 #define RAISE_EXCEPTION(s)                         \
   {                                                \
     ExceptionObject exception(__FILE__, __LINE__); \
@@ -253,7 +254,8 @@ PolygonGroupSpatialObjectXMLFileWriter::WriteFile()
     {
       PolygonSpatialObjectType::PointType curpoint = pointIt->GetPositionInObjectSpace();
       WriteStartElement("POINT", output);
-      output << curpoint[0] << ' ' << curpoint[1] << ' ' << curpoint[2];
+      output << itk::ConvertNumberToString(curpoint[0]) << ' ' << itk::ConvertNumberToString(curpoint[1]) << ' '
+             << itk::ConvertNumberToString(curpoint[2]);
       WriteEndElement("POINT", output);
       output << std::endl;
       ++pointIt;