Fields as underlying types

When you declare fields in a tuple or struct using field name (offset = X, type = T);, the generated C++ exposes helper wrapper types:

• rust::FieldOwned<T, OFFSET> for fields on owning types
• rust::FieldRef<T, OFFSET> for fields on Ref<Ty>
• rust::FieldRefMut<T, OFFSET> for fields on RefMut<Ty>

These wrappers now act as their underlying type T in many contexts:

• Ref<T> construction from any Field*<T, OFFSET>
• Implicit read via operator T() for value-like access
• Method calls are forwarded when applicable

Example:

rust::Tuple<int32_t, rust::std::string::String> t{42, "hi"_rs.to_owned()};

// Read value
int32_t v = t.f0; // operator T() on FieldOwned<int32_t, 0>

// Get a Ref<T> from a field
rust::Ref<int32_t> r = t.f0;

// Access methods through Ref from Field wrappers
rust::Ref<rust::std::string::String> sref = t.f1;
auto len = sref.len();

// From references to container, fields become FieldRef/FieldRefMut
rust::Ref<decltype(t)> rt = t;
auto l1 = rt.f1.len();

rust::RefMut<decltype(t)> mt = t;
mt.f1.push_str("!"_rs);

See examples/regression_test1 for a runnable demonstration.