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.hpp

@@ -0,0 +1,40 @@
+// Owns the video-mirror pipeline (WGC capture + frame renderer) and its UI.
+// The target window comes from the injected hook's reported game HWND.
+#pragma once
+
+#include <cstdint>
+
+#include <d3d11.h>
+#include <windows.h>
+
+#include "capture/frame_renderer.hpp"
+#include "capture/window_capture.hpp"
+
+namespace coop
+{
+
+class CapturePanel
+{
+public:
+	bool init(ID3D11Device* device);
+
+	// The window to mirror (0 if none yet); typically the injected game's HWND.
+	void set_target(HWND target)
+	{
+		target_ = target;
+	}
+
+	void draw_ui();
+
+	// Render thread: draw the mirrored frame as the window background.
+	void render(ID3D11DeviceContext* ctx, std::uint32_t dst_w, std::uint32_t dst_h);
+
+private:
+	ID3D11Device* device_ = nullptr;
+	FrameRenderer renderer_;
+	WindowCapture capture_;
+	HWND target_ = nullptr;
+	bool enabled_ = false;
+};
+
+} // namespace coop