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/main.cpp

@@ -1,6 +1,6 @@
 // CoopMockGame -- a tiny test "game" used to exercise the capture + audio + hook paths.
 //
-//   coop_mock_game.exe [dx9|dx9ex|dx10|dx11|dx12|gl] [seconds] [rate] [channels] [bits] [pcm|float]
+//   coop_mock_game.exe [dx9|dx9ex|dx10|dx11|dx12|gl|vk] [seconds] [rate] [channels] [bits] [pcm|float]
 //
 // It opens a normal visible window and renders an animated, frame-numbered pattern (see
 // render_backend.hpp): a moving bar + per-frame background colour make motion obvious, and