style: Declare data of Stream classes private, instead of protected

Following C++ Core Guidelines, Jul 8, 2025, "Avoid `protected` data",
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rh-protected

Author: N-Dekker

include/itkInputBinaryStream.h

@@ -89,7 +89,7 @@ class WebAssemblyInterface_EXPORT InputBinaryStream
     }
   }
 
-protected:
+private:
   std::istream * m_IStream{ nullptr };
   bool           m_DeleteIStream{ false };
 

include/itkInputTextStream.h

@@ -91,7 +91,7 @@ class WebAssemblyInterface_EXPORT InputTextStream
     }
   }
 
-protected:
+private:
   std::istream * m_IStream{ nullptr };
   bool           m_DeleteIStream{ false };
 

include/itkOutputBinaryStream.h

@@ -89,7 +89,7 @@ class WebAssemblyInterface_EXPORT OutputBinaryStream
     return this->m_Identifier;
   }
 
-protected:
+private:
   std::ostream * m_OStream{ nullptr };
   bool           m_DeleteOStream{ false };
 

include/itkOutputTextStream.h

@@ -89,7 +89,7 @@ class WebAssemblyInterface_EXPORT OutputTextStream
     return this->m_Identifier;
   }
 
-protected:
+private:
   std::ostream * m_OStream{ nullptr };
   bool           m_DeleteOStream{ false };
 

include/itkWasmStringStream.h

@@ -123,11 +123,12 @@ class WebAssemblyInterface_EXPORT WasmStringStream : public WasmDataObject
     this->m_JSON = jsonStream.str();
   }
 
-  std::stringstream m_StringStream;
-  std::string       m_String;
-
   void
   PrintSelf(std::ostream & os, Indent indent) const override;
+
+private:
+  std::stringstream m_StringStream;
+  std::string       m_String;
 };
 
 } // namespace itk