[WIP] Add stub implementations for interface member functions using C++26 reflection and vanishing-this-pointer

[Copilot]

Adds member-function-like call syntax to xyz::reflection::protocol<T> and protocol_view<T> via compile-time stub synthesis. The stubs are invocable with the correct signatures; bodies call std::unreachable() pending the vtable dispatch layer.

Approach

Vanishing-this-pointer thunks (reflection/protocol.h): Four thunk templates cover the {const, mutable} × {noexcept, throwing} matrix. Each thunk's operator() mirrors one interface method's exact signature and recovers the enclosing base pointer via static_cast<OwnerBase*>(static_cast<void*>(this)).

Reflection-driven synthesis: A consteval helper (detail::method_thunk_specs) iterates members_of(^^T), builds a function-pointer type R(*)(Args...) per method via substitute, selects the appropriate thunk template, and returns data_member_spec entries. Overloaded names are deduplicated (first declaration wins; a comment describes the encoding approach for a future revision).

define_aggregate in consteval blocks: protocol<T> and protocol_view<T> each inherit from a forward-declared base (protocol_member_stubs<T> / protocol_view_member_stubs<T>) that is completed by a consteval {} block in the class body, injecting one [[no_unique_address]] thunk per method.

struct Interface {
  std::string_view name() const noexcept;
  int count();
};

// Both calls compile with the correct return types and noexcept status.
static_assert(noexcept(std::declval<const protocol<Interface>>().name()));
static_assert(!noexcept(std::declval<protocol<Interface>>().count()));

conforms_to<Interface, Concrete>(): Special member functions are now explicitly skipped (previously relied on has_identifier as an imperfect guard).

Supporting changes

• reflection/CMakeLists.txt: build target (C++26, -freflection).
• CMakeLists.txt: XYZ_PROTOCOL_BUILD_REFLECTION_IMPLEMENTATION option with a check_cxx_source_compiles guard; add_subdirectory(reflection) placed inside the BUILD_TESTING block so GTest is available.
• scripts/cmake.py: --reflection flag; uses setdefault so existing CXX/CC environment variables take precedence over the g++-16 fallback.
• cmake/xyz_add_test.cmake: added 26 to the list of valid VERSION values.

• Fixes Add stub implementations for interface member functions #130