M2(DX9): dual-mode DX9 mock-game render backend

Add render_dx09.cpp (selectable as `dx9ex` / `dx9`): renders the animated
pattern with Clear + ColorFill (D3D9's built-in rect fill -- no shaders) and
presents through a real D3D9 / D3D9Ex device. Two modes so the two D3D9 capture
paths each have a matching game. Smoke-verified: both present frames and exit
cleanly. The capture hook + frame-decode test land next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/mock_game/render_backend.hpp

@@ -61,5 +61,6 @@ public:
 std::unique_ptr<RenderBackend> create_dx11_backend();
 std::unique_ptr<RenderBackend> create_dx12_backend();
 std::unique_ptr<RenderBackend> create_dx10_backend();
+std::unique_ptr<RenderBackend> create_dx9_backend(bool ex); // ex: D3D9Ex vs plain D3D9
 
 } // namespace coop::mock