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/CMakeLists.txt

@@ -4,10 +4,13 @@ add_executable(coop_host WIN32
 	src/imgui_layer.cpp
 	src/debug_overlay.cpp
 	src/injection_panel.cpp
+	src/capture_panel.cpp
 	src/input/xinput_source.cpp
 	src/inject/process_list.cpp
 	src/inject/injector.cpp
-	src/ipc/ipc_server.cpp)
+	src/ipc/ipc_server.cpp
+	src/capture/frame_renderer.cpp
+	src/capture/window_capture.cpp)
 
 target_include_directories(coop_host PRIVATE src)
 
@@ -17,6 +20,8 @@ target_link_libraries(coop_host PRIVATE
 	d3d11
 	dxgi
 	dwmapi
+	d3dcompiler
+	windowsapp
 	xinput)
 
 set_target_properties(coop_host PROPERTIES OUTPUT_NAME "coop_host")