Forward declare tuple_size/tuple_element and raw_json, move custom into forward.hpp

[stephenberry]

Follow-up to the glaze/forward.hpp work, addressing the additional types raised in review feedback on #2527.

What

• glz::tuple_size / glz::tuple_element — forward declare the primary templates (left incomplete) so user headers can register a tuple-like type via specialization without pulling in glaze/tuplet/tuple.hpp. The concrete tuple_size_v / tuple_element_t helpers and the std:: specializations remain owned by tuple.hpp.
• glz::raw_json — forward declare basic_raw_json plus the raw_json / raw_json_view aliases, supporting use as a default template argument in a generic lambda, e.g.:

  static constexpr auto value = our_glz_wrappers::custom<
    [](T& x, double v) { x = T(v); },
    []<typename raw_json = glz::raw_json>(T x) { return raw_json(our_custom_type_to_string(x)); }
  >;

  The declaration omits the default template argument (std::string), leaving glaze/core/common.hpp as the sole owner of the default and full definition.
• glz::custom — custom_impl() and the custom customization point move from glaze/core/wrappers.hpp into forward.hpp, next to custom_t (which already lived there). Both are trivial and depend only on custom_t.
• glz::error_code — intentionally left out; it doesn't appear to be used without its definition.

forward.hpp now also includes <cstddef>, <string>, and <string_view> alongside <cstdint>.

Verification

• forward.hpp standalone compiles and exposes the new names (raw_json as a default template arg, a user tuple_size/tuple_element specialization, glz::custom).
• forward.hpp + full glaze.hpp compiles in both include orders with no redefinition or default-argument clashes.