Add mock-game capture/audio/hook stress test

mock_game_test launches coop_mock_game (DX11/DX12, frame-numbered A/V), injects
coop_hook.dll, opens the hook's shared video texture, and decodes the frame
number from the captured pixels to assert a monotonic, advancing mirror for
both backends (catches dropped/stale/out-of-order frames). Then it runs an
A/V + hook/unhook stress pass and confirms no crash + capture resumes. Adds a
read_pixel() readback to the shipping SharedTextureSource for the decode.
Robust to repeated/CI runs (kills stray games, retries inject). Trim the
roadmap (the mock-game item is done) and document the test.

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

host/src/capture/shared_texture.cpp

@@ -92,6 +92,44 @@ bool SharedTextureSource::reopen(unsigned long pid, const VideoShareView& share)
 	return true;
 }
 
+bool SharedTextureSource::read_pixel(std::uint32_t x, std::uint32_t y, std::uint8_t out[4])
+{
+	if (private_ == nullptr || ctx_ == nullptr || device_ == nullptr)
+	{
+		return false;
+	}
+	D3D11_TEXTURE2D_DESC desc{};
+	private_->GetDesc(&desc);
+	if (x >= desc.Width || y >= desc.Height)
+	{
+		return false;
+	}
+	D3D11_TEXTURE2D_DESC staging_desc = desc;
+	staging_desc.Usage = D3D11_USAGE_STAGING;
+	staging_desc.BindFlags = 0;
+	staging_desc.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
+	staging_desc.MiscFlags = 0;
+	Microsoft::WRL::ComPtr<ID3D11Texture2D> staging;
+	if (FAILED(device_->CreateTexture2D(&staging_desc, nullptr, &staging)))
+	{
+		return false;
+	}
+	ctx_->CopyResource(staging.Get(), private_.Get());
+	D3D11_MAPPED_SUBRESOURCE map{};
+	if (FAILED(ctx_->Map(staging.Get(), 0, D3D11_MAP_READ, 0, &map)))
+	{
+		return false;
+	}
+	const auto* px = static_cast<const std::uint8_t*>(map.pData) + static_cast<std::size_t>(y) * map.RowPitch +
+	                 static_cast<std::size_t>(x) * 4; // R8G8B8A8_UNORM
+	out[0] = px[0];
+	out[1] = px[1];
+	out[2] = px[2];
+	out[3] = px[3];
+	ctx_->Unmap(staging.Get(), 0);
+	return true;
+}
+
 bool SharedTextureSource::update(const VideoShareView& share, unsigned long pid)
 {
 	if (device_ == nullptr || pid == 0)

host/src/capture/shared_texture.hpp

@@ -31,6 +31,12 @@ public:
 	// Drop the opened resources (target changed / mirror turned off).
 	void reset();
 
+	// Read the RGBA8 pixel at (x,y) of the last copied frame into out[4]. For verification
+	// (e.g. decoding the mock game's frame-number block in a capture test). Copies the
+	// private texture to a CPU-readable staging texture and maps it -- not a hot path.
+	// Returns false if no frame has been copied yet or the readback failed.
+	bool read_pixel(std::uint32_t x, std::uint32_t y, std::uint8_t out[4]);
+
 	[[nodiscard]] ID3D11ShaderResourceView* srv() const
 	{
 		return srv_.Get();