Phase 0: donor-launch spike foundation

Scaffold CoopAllTheThings: Remote Play Together for any XInput game via a
mirror app under a donor appid (real game keeps its own appid, so DRM,
achievements, and playtime stay intact).

- Build: CMake skeleton, ImGui + SafetyHook submodules (no vcpkg)
- common/: host<->hook IPC contract (seqlock pad state, shared-memory RAII)
- host/: borderless D3D11 window + ImGui overlay listing visible XInput pads,
  behind an InputSource interface (Steam Input slots in later)
- README documents the Phase 0 donor-launch validation procedure, anti-cheat
  limitation, and XInput/bitness constraints

Phase 0 validates the riskiest assumption (Steam RPT streams an arbitrary
window under a donor appid and routes guest input to it) before capture and
injection are built.

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

host/src/imgui_layer.cpp

@@ -0,0 +1,54 @@
+#include "imgui_layer.hpp"
+
+#include <imgui.h>
+#include <imgui_impl_dx11.h>
+#include <imgui_impl_win32.h>
+
+namespace coop
+{
+
+ImGuiLayer::~ImGuiLayer()
+{
+	if (initialized_)
+	{
+		ImGui_ImplDX11_Shutdown();
+		ImGui_ImplWin32_Shutdown();
+		ImGui::DestroyContext();
+		initialized_ = false;
+	}
+}
+
+bool ImGuiLayer::init(HWND hwnd, ID3D11Device* device, ID3D11DeviceContext* context)
+{
+	IMGUI_CHECKVERSION();
+	ImGui::CreateContext();
+	ImGuiIO& io = ImGui::GetIO();
+	io.IniFilename = nullptr; // don't litter the cwd with imgui.ini during the spike
+	ImGui::StyleColorsDark();
+
+	if (!ImGui_ImplWin32_Init(hwnd))
+	{
+		return false;
+	}
+	if (!ImGui_ImplDX11_Init(device, context))
+	{
+		return false;
+	}
+	initialized_ = true;
+	return true;
+}
+
+void ImGuiLayer::begin_frame()
+{
+	ImGui_ImplDX11_NewFrame();
+	ImGui_ImplWin32_NewFrame();
+	ImGui::NewFrame();
+}
+
+void ImGuiLayer::end_frame()
+{
+	ImGui::Render();
+	ImGui_ImplDX11_RenderDrawData(ImGui::GetDrawData());
+}
+
+} // namespace coop