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/CMakeLists.txt

@@ -0,0 +1,16 @@
+# CoopMockGame -- a tiny test "game" with an animated, frame-numbered render (DX11/DX12)
+# and a configurable WASAPI tone, used by the capture/audio/hook stress tests. Not shipped.
+add_executable(coop_mock_game
+	main.cpp
+	render_backend.cpp
+	render_dx11.cpp
+	render_dx12.cpp)
+
+# Reuses the shared ToneSource (also used by coop_tone + the audio hook self-test).
+target_include_directories(coop_mock_game PRIVATE ${CMAKE_SOURCE_DIR}/tools/audio_tone)
+
+target_link_libraries(coop_mock_game PRIVATE d3d11 d3d12 dxgi ole32)
+set_target_properties(coop_mock_game PROPERTIES OUTPUT_NAME "coop_mock_game")
+
+# Test fixture -> stage next to the tests (alongside coop_tone), not in the deployable root.
+coop_output_subdir(tests coop_mock_game)