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>

host/src/debug_overlay.cpp

@@ -0,0 +1,82 @@
+#include "debug_overlay.hpp"
+
+#include <imgui.h>
+
+namespace coop
+{
+
+namespace
+{
+
+struct ButtonBit
+{
+	std::uint16_t mask;
+	const char* label;
+};
+
+// XINPUT_GAMEPAD_* bit values (kept local so this file needn't include Xinput.h).
+constexpr ButtonBit kButtons[] = {
+    {0x0001, "Up"},   {0x0002, "Down"}, {0x0004, "Left"},  {0x0008, "Right"}, {0x0010, "Start"},
+    {0x0020, "Back"}, {0x0040, "LS"},   {0x0080, "RS"},    {0x0100, "LB"},    {0x0200, "RB"},
+    {0x1000, "A"},    {0x2000, "B"},    {0x4000, "X"},     {0x8000, "Y"},
+};
+
+void draw_pad(int index, const PadInfo& pad)
+{
+	ImGui::PushID(index);
+	if (!pad.connected)
+	{
+		ImGui::TextDisabled("Slot %d: disconnected", index);
+		ImGui::PopID();
+		return;
+	}
+
+	ImGui::Text("Slot %d  [%s]", index, pad.source.c_str());
+	ImGui::Text("LT %3u  RT %3u", pad.state.left_trigger, pad.state.right_trigger);
+	ImGui::Text("L (%6d, %6d)  R (%6d, %6d)", pad.state.thumb_lx, pad.state.thumb_ly, pad.state.thumb_rx,
+	            pad.state.thumb_ry);
+
+	bool first = true;
+	ImGui::TextUnformatted("Buttons: ");
+	for (const ButtonBit& b : kButtons)
+	{
+		if ((pad.state.buttons & b.mask) != 0)
+		{
+			ImGui::SameLine();
+			ImGui::TextColored(ImVec4(0.4f, 1.0f, 0.4f, 1.0f), "%s%s", first ? "" : ", ", b.label);
+			first = false;
+		}
+	}
+	if (first)
+	{
+		ImGui::SameLine();
+		ImGui::TextDisabled("(none)");
+	}
+	ImGui::Separator();
+	ImGui::PopID();
+}
+
+} // namespace
+
+void draw_debug_overlay(const InputSource& input)
+{
+	ImGui::SetNextWindowPos(ImVec2(24, 24), ImGuiCond_FirstUseEver);
+	ImGui::SetNextWindowSize(ImVec2(420, 0), ImGuiCond_FirstUseEver);
+	ImGui::Begin("CoopAllTheThings - Phase 0 spike");
+
+	ImGui::Text("Input backend: %s", input.name());
+	ImGui::Text("%.1f FPS (%.2f ms)", ImGui::GetIO().Framerate, 1000.0f / ImGui::GetIO().Framerate);
+	ImGui::TextDisabled("This window is what Remote Play Together captures.");
+	ImGui::TextDisabled("Press Esc to quit.");
+	ImGui::Separator();
+
+	const auto& pads = input.pads();
+	for (int i = 0; i < static_cast<int>(pads.size()); ++i)
+	{
+		draw_pad(i, pads[i]);
+	}
+
+	ImGui::End();
+}
+
+} // namespace coop