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/CMakeLists.txt

@@ -0,0 +1,18 @@
+add_executable(coop_host WIN32
+	src/main.cpp
+	src/d3d11_window.cpp
+	src/imgui_layer.cpp
+	src/debug_overlay.cpp
+	src/input/xinput_source.cpp)
+
+target_include_directories(coop_host PRIVATE src)
+
+target_link_libraries(coop_host PRIVATE
+	coop_common
+	imgui
+	d3d11
+	dxgi
+	dwmapi
+	xinput)
+
+set_target_properties(coop_host PROPERTIES OUTPUT_NAME "coop_host")