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>

common/include/coop/protocol.hpp

@@ -0,0 +1,118 @@
+// IPC contract shared between the host (coop_host.exe) and the injected hook
+// DLL (coop_hook.dll). Both sides compile this identical header, so the memory
+// layout must stay POD and version-locked.
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+
+namespace coop
+{
+
+// Bump whenever the layout of SharedBlock or CoopPadState changes. The hook
+// refuses to attach to a host with a mismatched version.
+inline constexpr std::uint32_t kProtocolVersion = 1;
+
+// 'COOP' little-endian, used to sanity-check the mapping before trusting it.
+inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
+
+// XInput exposes four controller slots; we mirror that fixed count.
+inline constexpr std::uint32_t kMaxPads = 4;
+
+// The shared-memory section is named per host process id so multiple sessions
+// can coexist. Format with the target game's pid: coop_ipc_<pid>.
+inline constexpr wchar_t kSharedMemoryPrefix[] = L"Local\\coop_ipc_";
+
+// One controller's state, laid out to map 1:1 onto XINPUT_GAMEPAD plus the
+// metadata the hook needs. Field names/types match XINPUT_GAMEPAD so the hook
+// can memcpy the trailing region straight into an XINPUT_STATE.
+struct CoopPadState
+{
+	std::uint8_t connected; // 1 if a guest/host pad is mapped to this slot
+	std::uint8_t reserved[3];
+	std::uint32_t packet;   // bumps on change -> XINPUT_STATE::dwPacketNumber
+	std::uint16_t buttons;  // XINPUT_GAMEPAD_* bitmask
+	std::uint8_t left_trigger;
+	std::uint8_t right_trigger;
+	std::int16_t thumb_lx;
+	std::int16_t thumb_ly;
+	std::int16_t thumb_rx;
+	std::int16_t thumb_ry;
+};
+
+static_assert(sizeof(CoopPadState) == 20, "CoopPadState layout must stay stable across both modules");
+
+// Top-level shared block. The host is the sole writer of pad state; the hook is
+// the sole reader. A seqlock (even = stable, odd = write in progress) lets the
+// reader grab a torn-free snapshot without a kernel lock on the hot path.
+struct SharedBlock
+{
+	std::uint32_t magic;
+	std::uint32_t version;
+	std::uint32_t pad_count; // number of populated slots, <= kMaxPads
+	std::atomic<std::uint32_t> sequence;
+	CoopPadState pads[kMaxPads];
+
+	// Phase 2 appends the shared-texture handle/dimensions control fields here;
+	// keep new members at the end so existing offsets never shift.
+};
+
+static_assert(std::atomic<std::uint32_t>::is_always_lock_free,
+              "seqlock requires a lock-free 32-bit atomic for cross-process use");
+
+// --- Seqlock helpers -------------------------------------------------------
+
+// Writer side: publish a fresh set of pad states. Called from the host.
+inline void publish_pads(SharedBlock& block, const CoopPadState* pads, std::uint32_t count)
+{
+	if (count > kMaxPads)
+	{
+		count = kMaxPads;
+	}
+	const std::uint32_t seq = block.sequence.load(std::memory_order_relaxed);
+	block.sequence.store(seq + 1, std::memory_order_release); // -> odd: write begins
+	std::atomic_thread_fence(std::memory_order_release);
+	block.pad_count = count;
+	for (std::uint32_t i = 0; i < count; ++i)
+	{
+		block.pads[i] = pads[i];
+	}
+	for (std::uint32_t i = count; i < kMaxPads; ++i)
+	{
+		block.pads[i] = CoopPadState{};
+	}
+	block.sequence.store(seq + 2, std::memory_order_release); // -> even: write done
+}
+
+// Reader side: copy a consistent snapshot. Called from the hook. Spins briefly
+// if a write is in flight; bounded so a crashed writer can't hang the game.
+inline bool read_pads(const SharedBlock& block, CoopPadState (&out)[kMaxPads], std::uint32_t& out_count)
+{
+	for (int attempt = 0; attempt < 64; ++attempt)
+	{
+		const std::uint32_t before = block.sequence.load(std::memory_order_acquire);
+		if (before & 1u)
+		{
+			continue; // writer mid-update, retry
+		}
+		std::uint32_t count = block.pad_count;
+		if (count > kMaxPads)
+		{
+			count = kMaxPads;
+		}
+		for (std::uint32_t i = 0; i < kMaxPads; ++i)
+		{
+			out[i] = block.pads[i];
+		}
+		std::atomic_thread_fence(std::memory_order_acquire);
+		const std::uint32_t after = block.sequence.load(std::memory_order_acquire);
+		if (before == after)
+		{
+			out_count = count;
+			return true;
+		}
+	}
+	return false;
+}
+
+} // namespace coop

common/include/coop/shared_memory.hpp

@@ -0,0 +1,131 @@
+// Thin RAII wrapper over a Win32 file-mapping section used as the host<->hook
+// IPC transport. Header-only so both modules share one implementation.
+#pragma once
+
+#include <string>
+#include <utility>
+
+#include <windows.h>
+
+#include "coop/protocol.hpp"
+
+namespace coop
+{
+
+class SharedMemory
+{
+public:
+	SharedMemory() = default;
+
+	SharedMemory(const SharedMemory&) = delete;
+	SharedMemory& operator=(const SharedMemory&) = delete;
+
+	SharedMemory(SharedMemory&& other) noexcept
+	{
+		*this = std::move(other);
+	}
+
+	SharedMemory& operator=(SharedMemory&& other) noexcept
+	{
+		if (this != &other)
+		{
+			reset();
+			mapping_ = std::exchange(other.mapping_, nullptr);
+			view_ = std::exchange(other.view_, nullptr);
+			size_ = std::exchange(other.size_, 0);
+		}
+		return *this;
+	}
+
+	~SharedMemory()
+	{
+		reset();
+	}
+
+	// Host side: create (or open if it already exists) the named section.
+	bool create(const std::wstring& name, std::size_t size)
+	{
+		reset();
+		mapping_ = CreateFileMappingW(INVALID_HANDLE_VALUE, nullptr, PAGE_READWRITE, 0,
+		                              static_cast<DWORD>(size), name.c_str());
+		if (mapping_ == nullptr)
+		{
+			return false;
+		}
+		return map(size);
+	}
+
+	// Hook side: open an existing section created by the host.
+	bool open(const std::wstring& name, std::size_t size)
+	{
+		reset();
+		mapping_ = OpenFileMappingW(FILE_MAP_ALL_ACCESS, FALSE, name.c_str());
+		if (mapping_ == nullptr)
+		{
+			return false;
+		}
+		return map(size);
+	}
+
+	void reset()
+	{
+		if (view_ != nullptr)
+		{
+			UnmapViewOfFile(view_);
+			view_ = nullptr;
+		}
+		if (mapping_ != nullptr)
+		{
+			CloseHandle(mapping_);
+			mapping_ = nullptr;
+		}
+		size_ = 0;
+	}
+
+	[[nodiscard]] bool valid() const
+	{
+		return view_ != nullptr;
+	}
+
+	template <typename T>
+	[[nodiscard]] T* as() const
+	{
+		return static_cast<T*>(view_);
+	}
+
+	[[nodiscard]] void* data() const
+	{
+		return view_;
+	}
+
+	[[nodiscard]] std::size_t size() const
+	{
+		return size_;
+	}
+
+private:
+	bool map(std::size_t size)
+	{
+		view_ = MapViewOfFile(mapping_, FILE_MAP_ALL_ACCESS, 0, 0, size);
+		if (view_ == nullptr)
+		{
+			CloseHandle(mapping_);
+			mapping_ = nullptr;
+			return false;
+		}
+		size_ = size;
+		return true;
+	}
+
+	HANDLE mapping_ = nullptr;
+	void* view_ = nullptr;
+	std::size_t size_ = 0;
+};
+
+// Build the per-pid section name both sides agree on.
+inline std::wstring shared_memory_name(unsigned long target_pid)
+{
+	return std::wstring(kSharedMemoryPrefix) + std::to_wstring(target_pid);
+}
+
+} // namespace coop