Changes

• Updated the wording for "pointer-like type eligible for proxy" in named requirements ProFacade: Removed clause "P is not a specialization of proxy". This does not align with the standard wording style.
• In the value constructors / assignments of proxy, specified that they won't participate overload resolution if std::decay_t<P> is a specialization of proxy.
• Split detail::ptr_traits into concept pointer_like and concept specialization_of.
• Added missing requires clause in swap().
• Removed redundant <F> from invoke().
• Added a unit test case for proxiable eligibility.