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.cpp

@@ -0,0 +1,54 @@
+#include "ipc/ipc_server.hpp"
+
+namespace coop
+{
+
+bool IpcServer::start(unsigned long target_pid)
+{
+	stop();
+
+	if (!shm_.create(shared_memory_name(target_pid), sizeof(SharedBlock)))
+	{
+		return false;
+	}
+
+	auto* block = shm_.as<SharedBlock>();
+	// Fresh CreateFileMapping pages are zero-filled; set the header before the
+	// hook (which validates magic/version) has a chance to read it.
+	block->version = kProtocolVersion;
+	block->pad_count = 0;
+	block->sequence.store(0, std::memory_order_relaxed);
+	block->magic = kProtocolMagic; // publish magic last so a racing reader bails
+
+	block_ = block;
+	target_pid_ = target_pid;
+	return true;
+}
+
+void IpcServer::publish(const std::array<PadInfo, kMaxPads>& pads)
+{
+	if (block_ == nullptr)
+	{
+		return;
+	}
+	CoopPadState states[kMaxPads];
+	for (std::size_t i = 0; i < pads.size(); ++i)
+	{
+		states[i] = pads[i].state;
+		states[i].connected = pads[i].connected ? 1 : 0;
+	}
+	publish_pads(*block_, states, kMaxPads);
+}
+
+void IpcServer::stop()
+{
+	if (block_ != nullptr)
+	{
+		block_->magic = 0; // invalidate so a late hook read won't trust stale data
+		block_ = nullptr;
+	}
+	shm_.reset();
+	target_pid_ = 0;
+}
+
+} // namespace coop