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>

tests/mock_game_test.cpp

@@ -300,6 +300,37 @@ void test_video_capture(const char* backend, ID3D11Device* device)
 	game.kill();
 }
 
+// Liveness smoke check for a backend whose capture can't be exercised by late injection
+// (Vulkan caches its present pointer at init): launch it for a couple of seconds and assert it
+// comes up and exits cleanly. Exit code 2 = backend unavailable on this machine (e.g. no Vulkan
+// driver) -> skip without failing.
+void test_liveness(const char* backend)
+{
+	std::printf("== liveness: %s ==\n", backend);
+	std::wstring args;
+	for (const char* p = backend; *p != '\0'; ++p)
+	{
+		args.push_back(static_cast<wchar_t>(*p));
+	}
+	args += L" 2"; // run 2s then exit on its own
+	MockGame game = MockGame::launch(args);
+	if (!game.ok)
+	{
+		check(false, "launch coop_mock_game (liveness)");
+		return;
+	}
+	WaitForSingleObject(game.pi.hProcess, 6000); // let the 2s run finish
+	if (!game.alive() && game.exit_code() == 2)
+	{
+		std::printf("   backend '%s' unavailable on this machine -- skipping\n", backend);
+		game.kill();
+		return;
+	}
+	check(!game.alive(), "backend ran and exited within the time limit (no hang)");
+	check(game.exit_code() == 0, "backend came up and exited cleanly");
+	game.kill();
+}
+
 // Launch the mock game rendering audio at `rate`/`channels`/`bits`/`fmt`, inject the audio
 // hook (late, so it's the guessed path), and verify the hook MEASURES the right sample rate
 // for this variant and captures non-silent audio. (Channels/bit-depth aren't recoverable for
@@ -522,6 +553,10 @@ int main()
 		return 0;
 	}
 
+	// Vulkan: its present pointer is cached at init, so late injection can't hook it -- a
+	// liveness check (the mock comes up + presents + exits) is the automated coverage here; the
+	// capture path is exercised via the early-load path documented in the README.
+	test_liveness("vk");
 	test_video_capture("gl", device);
 	test_video_capture("dx9ex", device);
 	test_video_capture("dx9", device);