Move 'arg' variable to detail namespace (issue #150)

[Copilot]

The arg variable is an implementation detail used as the default placeholder text for option arguments in help messages. It was incorrectly exposed in the public boost::program_options namespace.

Changes

• include/boost/program_options/detail/value_semantic.hpp: Moved extern declaration into detail namespace; updated reference in typed_value<T, charT>::name()
• src/value_semantic.cpp: Moved definition into detail namespace; updated reference in untyped_value::name()

Before:

namespace boost { namespace program_options {
    extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
    // ...
    std::string const& var = (m_value_name.empty() ? arg : m_value_name);
}}

After:

namespace boost { namespace program_options {
    namespace detail {
        extern BOOST_PROGRAM_OPTIONS_DECL std::string arg;
    }
    // ...
    std::string const& var = (m_value_name.empty() ? detail::arg : m_value_name);
}}

Original prompt

in the detail/value_semantic.hpp, move the 'arg' variable to 'detail' sub-namespace. Adjust value_semantic.cpp to match. In the PR, mention issue #150

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