M2(Vulkan): Vulkan mock-game backend + volk/Vulkan-Headers submodules

Add render_vk.cpp (selectable as `vk`): brings up a real Vulkan
instance/device/swap chain via volk (which dlopens vulkan-1.dll -- the
loader-bypass case the capture hook must handle) and clears the swap-chain image
to the frame-counter colour each frame with vkCmdClearColorImage (no pipeline,
no shaders, no SPIR-V) and presents. The whole image encodes the frame number,
so it animates and stale frames are detectable.

Adds the official Khronos Vulkan-Headers + zeux/volk submodules and a
coop_require_submodule() CMake helper that fails with a clear "git submodule
update --init --recursive" message rather than auto-cloning. volk is pinned to
the project's dynamic CRT (no LNK4098).

mock_game_test gets a vk liveness check (its present pointer is cached at init,
so late injection can't hook it -- the capture path needs the early-load path,
to come). 16/16 ctest, skips cleanly without a Vulkan driver.

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

tools/mock_game/CMakeLists.txt

@@ -7,12 +7,13 @@ add_executable(coop_mock_game
 	render_dx12.cpp
 	render_dx10.cpp
 	render_dx09.cpp
-	render_gl.cpp)
+	render_gl.cpp
+	render_vk.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 d3d10 d3d9 dxgi ole32 opengl32 gdi32)
+target_link_libraries(coop_mock_game PRIVATE d3d11 d3d12 d3d10 d3d9 dxgi ole32 opengl32 gdi32 volk)
 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.