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/d3d11_window.hpp

@@ -0,0 +1,68 @@
+// Borderless full-screen D3D11 window. This is the surface Steam Remote Play
+// Together captures, so it must be a normal (non-exclusive) top-level window
+// that owns a flip-model swap chain.
+#pragma once
+
+#include <d3d11.h>
+#include <dxgi1_2.h>
+#include <wrl/client.h>
+
+#include <functional>
+
+namespace coop
+{
+
+class D3D11Window
+{
+public:
+	using RenderCallback = std::function<void()>;
+
+	D3D11Window() = default;
+	~D3D11Window();
+
+	D3D11Window(const D3D11Window&) = delete;
+	D3D11Window& operator=(const D3D11Window&) = delete;
+
+	// Creates the borderless window sized to the primary monitor and brings up
+	// the D3D11 device + swap chain. Returns false on any failure.
+	bool create(const wchar_t* title);
+
+	// Pumps the message queue once. Returns false when the window is closing.
+	bool pump_messages();
+
+	// Clears the back buffer, invokes render (where ImGui draws), and presents.
+	void render_frame(const RenderCallback& render);
+
+	[[nodiscard]] HWND hwnd() const
+	{
+		return hwnd_;
+	}
+	[[nodiscard]] ID3D11Device* device() const
+	{
+		return device_.Get();
+	}
+	[[nodiscard]] ID3D11DeviceContext* context() const
+	{
+		return context_.Get();
+	}
+
+private:
+	static LRESULT CALLBACK wnd_proc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam);
+
+	bool create_device();
+	void create_render_target();
+	void release_render_target();
+	void handle_resize(UINT width, UINT height);
+
+	HWND hwnd_ = nullptr;
+	bool resize_pending_ = false;
+	UINT resize_width_ = 0;
+	UINT resize_height_ = 0;
+
+	Microsoft::WRL::ComPtr<ID3D11Device> device_;
+	Microsoft::WRL::ComPtr<ID3D11DeviceContext> context_;
+	Microsoft::WRL::ComPtr<IDXGISwapChain1> swap_chain_;
+	Microsoft::WRL::ComPtr<ID3D11RenderTargetView> rtv_;
+};
+
+} // namespace coop