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>

CMakeLists.txt

@@ -0,0 +1,36 @@
+cmake_minimum_required(VERSION 3.21)
+
+project(CoopAllTheThings
+	VERSION 0.0.1
+	DESCRIPTION "Steam Remote Play Together for any XInput game"
+	LANGUAGES CXX)
+
+if(NOT WIN32)
+	message(FATAL_ERROR "CoopAllTheThings targets Windows only.")
+endif()
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+# Single output dir keeps the donor-launch / inject workflow simple: host exe and
+# hook dll land next to each other under /bin (already in .gitignore).
+set(COOP_OUTPUT_DIR "${CMAKE_SOURCE_DIR}/bin/$<CONFIG>")
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${COOP_OUTPUT_DIR}")
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${COOP_OUTPUT_DIR}")
+
+if(MSVC)
+	add_compile_options(/W4 /permissive- /Zc:__cplusplus /utf-8 /MP)
+	add_compile_definitions(UNICODE _UNICODE WIN32_LEAN_AND_MEAN NOMINMAX)
+endif()
+
+add_subdirectory(third_party)
+add_subdirectory(common)
+add_subdirectory(host)
+
+# The injected hook DLL pulls in SafetyHook (+ Zydis). It is built in Phase 1;
+# enable once the dependency is wired so Phase 0 stays minimal.
+option(COOP_BUILD_HOOK "Build the injected game-side hook DLL" OFF)
+if(COOP_BUILD_HOOK)
+	add_subdirectory(hook)
+endif()