Add coop_mock_game: animated, frame-numbered A/V test game (DX11 + DX12)

A tiny test "game" for exercising the capture/audio/hook paths. Opens a normal
window and renders an animated pattern -- moving bar + per-frame background so
motion (and dropped frames) are obvious -- with a top-left block whose RGB
encodes the exact frame number, so a capture test can decode it and detect
dropped / duplicated / stale frames. Selectable backend (dx11 / dx12 today),
behind a RenderBackend interface so OpenGL/Vulkan can be added. With audio args
it also plays a configurable WASAPI tone (shared ToneSource), so it's a full A/V
source with a window (unlike coop_tone).

  coop_mock_game.exe [dx11|dx12] [seconds] [rate] [channels] [bits] [pcm|float]

Milestone 1 of the mock-game roadmap item (the stress-test suite follows).

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

tools/mock_game/render_backend.cpp

@@ -0,0 +1,19 @@
+#include "render_backend.hpp"
+
+namespace coop::mock
+{
+
+std::unique_ptr<RenderBackend> RenderBackend::create(const std::string& name)
+{
+	if (name == "dx11")
+	{
+		return create_dx11_backend();
+	}
+	if (name == "dx12")
+	{
+		return create_dx12_backend();
+	}
+	return nullptr;
+}
+
+} // namespace coop::mock