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/frame_renderer.hpp

@@ -0,0 +1,30 @@
+// Draws a captured frame texture into the current render target as a letterboxed
+// full-screen image (aspect-preserved; the cleared background fills the bars).
+#pragma once
+
+#include <cstdint>
+
+#include <d3d11.h>
+#include <wrl/client.h>
+
+namespace coop
+{
+
+class FrameRenderer
+{
+public:
+	bool init(ID3D11Device* device);
+
+	// Draws `srv` (a srcW x srcH image) centered and scaled to fit within a
+	// dstW x dstH target. Sets its own viewport; callers that draw afterwards
+	// (e.g. ImGui) should restore theirs.
+	void draw(ID3D11DeviceContext* ctx, ID3D11ShaderResourceView* srv, std::uint32_t src_w, std::uint32_t src_h,
+	          std::uint32_t dst_w, std::uint32_t dst_h);
+
+private:
+	Microsoft::WRL::ComPtr<ID3D11VertexShader> vs_;
+	Microsoft::WRL::ComPtr<ID3D11PixelShader> ps_;
+	Microsoft::WRL::ComPtr<ID3D11SamplerState> sampler_;
+};
+
+} // namespace coop