M2: DX10 mock-game render backend

Add render_dx10.cpp (selectable as `dx10`): composes the animated pattern
(background + moving bar + top-left frame-counter block) on the CPU each frame
and CopyResource's it into a real D3D10 DXGI swap-chain back buffer (DX10 has no
rect-clear; no shaders). Smoke-verified: `coop_mock_game dx10 2` presents frames
and exits cleanly. The frame-accurate decode-through-the-hook assertion lands
with the DX10 capture commit next.

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

tools/mock_game/render_backend.hpp

@@ -60,5 +60,6 @@ public:
 // Per-backend factories (defined in their own TUs; create() dispatches to them).
 std::unique_ptr<RenderBackend> create_dx11_backend();
 std::unique_ptr<RenderBackend> create_dx12_backend();
+std::unique_ptr<RenderBackend> create_dx10_backend();
 
 } // namespace coop::mock