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.hpp

@@ -0,0 +1,27 @@
+// Owns ImGui setup/teardown for the Win32 + DX11 backends.
+#pragma once
+
+#include <d3d11.h>
+#include <windows.h>
+
+namespace coop
+{
+
+class ImGuiLayer
+{
+public:
+	ImGuiLayer() = default;
+	~ImGuiLayer();
+
+	ImGuiLayer(const ImGuiLayer&) = delete;
+	ImGuiLayer& operator=(const ImGuiLayer&) = delete;
+
+	bool init(HWND hwnd, ID3D11Device* device, ID3D11DeviceContext* context);
+	void begin_frame();
+	void end_frame();
+
+private:
+	bool initialized_ = false;
+};
+
+} // namespace coop