Move 'arg' variable to detail namespace (#151)

Closes #150.

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Vladimir Prus <vladimir.prus@gmail.com>

include/boost/program_options/detail/value_semantic.hpp

@@ -17,13 +17,15 @@ namespace boost { template<class T> class optional; }
 
 namespace boost { namespace program_options { 
 
-    extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
+    namespace detail {
+        extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
+    }
     
     template<class T, class charT>
     std::string
     typed_value<T, charT>::name() const
     {
-        std::string const& var = (m_value_name.empty() ? arg : m_value_name);
+        std::string const& var = (m_value_name.empty() ? detail::arg : m_value_name);
         if (!m_implicit_value.empty() && !m_implicit_value_as_text.empty()) {
             std::string msg = "[=" + var + "(=" + m_implicit_value_as_text + ")]";
             if (!m_default_value.empty() && !m_default_value_as_text.empty())

src/value_semantic.cpp

@@ -84,12 +84,14 @@ namespace boost { namespace program_options {
     }
 #endif
 
-    BOOST_PROGRAM_OPTIONS_DECL std::string arg("arg");
+    namespace detail {
+        BOOST_PROGRAM_OPTIONS_DECL std::string arg("arg");
+    }
 
     std::string
     untyped_value::name() const
     {
-        return arg;
+        return detail::arg;
     }
     
     unsigned