Phase 1b: video mirror via Windows Graphics Capture

Mirror the injected game's window into the host's borderless window so Steam
RPT streams a live copy of the game. No injection needed for video.

- capture/window_capture: WGC capture of a target HWND. Wraps our D3D11 device
  as an IDirect3DDevice, creates a free-threaded frame pool + capture session,
  hides the cursor and (best-effort) the capture border. Frames arrive on a WGC
  thread and are handed to the render thread, which copies the newest into a
  shader-resource texture and draws it -- keeping all D3D11 context use on one
  thread. Handles window resize via frame-pool Recreate.
- capture/frame_renderer: fullscreen-triangle shader that blits the captured
  texture letterboxed (aspect-preserved) into the window.
- capture_panel: "Mirror game window" toggle + lifecycle; target HWND comes
  from the hook's reported game window.
- main: winrt apartment init; mirrored frame drawn as background, ImGui on top.
- CMake: link windowsapp + d3dcompiler.

Verified: builds clean; host starts (apartment init + shader compile succeed).
Visual capture quality/latency to be judged on a real game.

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

host/src/capture_panel.cpp

@@ -0,0 +1,63 @@
+#include "capture_panel.hpp"
+
+#include "imgui.h"
+
+namespace coop
+{
+
+bool CapturePanel::init(ID3D11Device* device)
+{
+	device_ = device;
+	return renderer_.init(device);
+}
+
+void CapturePanel::draw_ui()
+{
+	ImGui::SetNextWindowPos(ImVec2(460, 24), ImGuiCond_FirstUseEver);
+	ImGui::SetNextWindowSize(ImVec2(360, 0), ImGuiCond_FirstUseEver);
+	ImGui::Begin("Video mirror");
+
+	const bool have_target = target_ != nullptr && IsWindow(target_);
+	ImGui::BeginDisabled(!have_target);
+	if (ImGui::Checkbox("Mirror game window", &enabled_))
+	{
+		if (!enabled_)
+		{
+			capture_.stop();
+		}
+	}
+	ImGui::EndDisabled();
+	if (!have_target)
+	{
+		ImGui::TextDisabled("Inject into a game first (its window is the source).");
+	}
+
+	// Start/restart capture when enabled and the target window changes.
+	if (enabled_ && have_target && capture_.target() != target_)
+	{
+		if (!capture_.start(target_, device_))
+		{
+			enabled_ = false;
+			ImGui::TextColored(ImVec4(1.0f, 0.45f, 0.4f, 1.0f), "Failed to start capture.");
+		}
+	}
+
+	if (capture_.running())
+	{
+		ImGui::TextColored(ImVec4(0.4f, 1.0f, 0.4f, 1.0f), "Capturing %ux%u", capture_.frame_width(),
+		                   capture_.frame_height());
+		ImGui::Text("Render: %.1f FPS", ImGui::GetIO().Framerate);
+	}
+
+	ImGui::End();
+}
+
+void CapturePanel::render(ID3D11DeviceContext* ctx, std::uint32_t dst_w, std::uint32_t dst_h)
+{
+	if (capture_.running())
+	{
+		capture_.draw_latest(renderer_, ctx, dst_w, dst_h);
+	}
+}
+
+} // namespace coop