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/injection_panel.hpp

@@ -6,6 +6,8 @@
 #include <string>
 #include <vector>
 
+#include <windows.h>
+
 #include "coop/protocol.hpp"
 #include "imgui.h"
 #include "inject/process_list.hpp"
@@ -25,6 +27,12 @@ public:
 	// test-input mode is on, a synthetic pattern is sent instead of `pads`.
 	void publish(const std::array<PadInfo, kMaxPads>& pads);
 
+	// The injected game's main window, as reported by the hook (null if none).
+	[[nodiscard]] HWND game_hwnd() const
+	{
+		return reinterpret_cast<HWND>(server_.hook_status().game_hwnd);
+	}
+
 private:
 	void refresh_processes();
 	void inject_selected();