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/injection_panel.hpp

@@ -0,0 +1,44 @@
+// ImGui panel that drives the input-forwarding pipeline: pick a target process,
+// inject coop_hook.dll, and stream pad state to it over shared memory.
+#pragma once
+
+#include <array>
+#include <string>
+#include <vector>
+
+#include "coop/protocol.hpp"
+#include "imgui.h"
+#include "inject/process_list.hpp"
+#include "ipc/ipc_server.hpp"
+
+namespace coop
+{
+
+class InjectionPanel
+{
+public:
+	InjectionPanel();
+
+	void draw();
+
+	// Forward the latest pad snapshot to the injected hook (if connected).
+	void publish(const std::array<PadInfo, kMaxPads>& pads)
+	{
+		server_.publish(pads);
+	}
+
+private:
+	void refresh_processes();
+	void inject_selected();
+
+	std::vector<ProcessEntry> processes_;
+	char filter_[128] = {};
+	unsigned long selected_pid_ = 0;
+	std::wstring selected_name_;
+
+	IpcServer server_;
+	std::string status_;
+	ImVec4 status_color_;
+};
+
+} // namespace coop