fix: detect virtual inheritance in add_base to prevent pointer offset crash (#6017)

Virtual inheritance places the base subobject at a dynamic offset, but
load_impl Case 2a uses reinterpret_cast which assumes a fixed offset.
This caused segfaults when dispatching inherited methods through virtual
bases (e.g. SftVirtDerived2::name()).

Add an is_static_downcastable SFINAE trait that detects whether
static_cast<Derived*>(Base*) is valid. When it is not (virtual
inheritance), set multiple_inheritance = true in add_base to force the
implicit_casts path, which correctly adjusts pointers at runtime.

Remove the workaround .def("name", &SftVirtDerived2::name) from
test_smart_ptr.cpp that was papering over the issue.

Made-with: Cursor

tests/test_smart_ptr.cpp

@@ -553,10 +553,6 @@ TEST_SUBMODULE(smart_ptr, m) {
     py::class_<SftVirtDerived2, SftVirtDerived, std::shared_ptr<SftVirtDerived2>>(
         m, "SftVirtDerived2")
         .def(py::init<>(&SftVirtDerived2::create))
-        // TODO: Remove this once inherited methods work through virtual bases.
-        //       Without it, d2.name() segfaults because pybind11 uses an incorrect
-        //       pointer offset when dispatching through the virtual inheritance chain.
-        .def("name", &SftVirtDerived2::name)
         .def("call_name", &SftVirtDerived2::call_name, py::arg("d2"));
 
     // test_move_only_holder