Phase 1a: input forwarding via DLL injection + XInput hook

The host can now inject coop_hook.dll into a running game and forward
controller state to it over shared memory, so the game reads the host's
(eventually the guest's) input and nothing else.

- hook/: coop_hook.dll. DllMain spawns a worker that opens the shared-memory
  channel (named by the game's pid) and installs SafetyHook inline hooks on
  XInputGetState/GetStateEx/GetCapabilities/SetState. Detours synthesize state
  from shared memory; unmanaged slots report disconnected, hiding physical pads.
- host/: process picker (Toolhelp32), CreateRemoteThread(LoadLibraryW) injector
  with an IsWow64Process2 bitness guard, IPC server publishing pads each frame,
  and an ImGui Injection panel wiring it together.
- tests/: hook_selftest exercises the IPC seqlock + hook detours in-process
  (no game/controller needed); passes.

Build: SafetyHook wired in (COOP_BUILD_HOOK=ON), Zydis via FetchContent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/ipc/ipc_server.hpp

@@ -0,0 +1,41 @@
+// Host side of the IPC channel: creates the shared-memory section (named by the
+// target game's pid) and publishes the forwarded controller state each frame.
+#pragma once
+
+#include <array>
+
+#include "coop/protocol.hpp"
+#include "coop/shared_memory.hpp"
+#include "input/input_source.hpp"
+
+namespace coop
+{
+
+class IpcServer
+{
+public:
+	// Creates and initializes the section for `target_pid`. The injected hook
+	// derives the same name from its own pid and opens it.
+	bool start(unsigned long target_pid);
+
+	// Pushes the current pad snapshot to the hook. No-op if not started.
+	void publish(const std::array<PadInfo, kMaxPads>& pads);
+
+	void stop();
+
+	[[nodiscard]] bool running() const
+	{
+		return block_ != nullptr;
+	}
+	[[nodiscard]] unsigned long target_pid() const
+	{
+		return target_pid_;
+	}
+
+private:
+	SharedMemory shm_;
+	SharedBlock* block_ = nullptr;
+	unsigned long target_pid_ = 0;
+};
+
+} // namespace coop