M2: DX10 mock-game render backend

Add render_dx10.cpp (selectable as `dx10`): composes the animated pattern
(background + moving bar + top-left frame-counter block) on the CPU each frame
and CopyResource's it into a real D3D10 DXGI swap-chain back buffer (DX10 has no
rect-clear; no shaders). Smoke-verified: `coop_mock_game dx10 2` presents frames
and exits cleanly. The frame-accurate decode-through-the-hook assertion lands
with the DX10 capture commit next.

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

tools/mock_game/render_dx10.cpp

@@ -0,0 +1,129 @@
+// DX10 backend for the mock game. DX10 has no rect-clear (ClearView), so it composes the
+// animated pattern (background + moving bar + top-left frame-counter block) into a CPU buffer
+// each frame, uploads it into a scratch texture, and CopyResource's that into the swap-chain
+// back buffer -- no shaders. It presents through a real D3D10 DXGI swap chain, so the capture
+// hook sees a genuine D3D10 present.
+#include "render_backend.hpp"
+
+#include <vector>
+
+#include <d3d10.h>
+#include <dxgi.h>
+#include <wrl/client.h>
+
+using Microsoft::WRL::ComPtr;
+
+namespace coop::mock
+{
+namespace
+{
+std::uint32_t pack(std::uint8_t r, std::uint8_t g, std::uint8_t b, std::uint8_t a = 255)
+{
+	// R8G8B8A8_UNORM byte order: R in the low byte.
+	return static_cast<std::uint32_t>(r) | (static_cast<std::uint32_t>(g) << 8) |
+	       (static_cast<std::uint32_t>(b) << 16) | (static_cast<std::uint32_t>(a) << 24);
+}
+
+class Dx10Backend : public RenderBackend
+{
+public:
+	bool init(HWND hwnd, std::uint32_t width, std::uint32_t height) override
+	{
+		width_ = width;
+		height_ = height;
+		px_.resize(static_cast<std::size_t>(width) * height);
+
+		DXGI_SWAP_CHAIN_DESC desc = {};
+		desc.BufferDesc.Width = width;
+		desc.BufferDesc.Height = height;
+		desc.BufferDesc.Format = DXGI_FORMAT_R8G8B8A8_UNORM; // UNORM (not sRGB) so the frame code is exact
+		desc.BufferDesc.RefreshRate.Numerator = 60;
+		desc.BufferDesc.RefreshRate.Denominator = 1;
+		desc.SampleDesc.Count = 1;
+		desc.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
+		desc.BufferCount = 2;
+		desc.OutputWindow = hwnd;
+		desc.Windowed = TRUE;
+		desc.SwapEffect = DXGI_SWAP_EFFECT_DISCARD; // blt model: GetBuffer(0) is the back buffer
+
+		if (FAILED(D3D10CreateDeviceAndSwapChain(nullptr, D3D10_DRIVER_TYPE_HARDWARE, nullptr, 0,
+		                                         D3D10_SDK_VERSION, &desc, swap_.GetAddressOf(),
+		                                         device_.GetAddressOf())))
+		{
+			return false;
+		}
+
+		// Scratch texture matching the back buffer; we upload the CPU frame into it then
+		// CopyResource it into the back buffer (DX10 can't fill a sub-rect of an RTV directly).
+		D3D10_TEXTURE2D_DESC td = {};
+		td.Width = width;
+		td.Height = height;
+		td.MipLevels = 1;
+		td.ArraySize = 1;
+		td.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
+		td.SampleDesc.Count = 1;
+		td.Usage = D3D10_USAGE_DEFAULT;
+		td.BindFlags = D3D10_BIND_SHADER_RESOURCE;
+		return SUCCEEDED(device_->CreateTexture2D(&td, nullptr, scratch_.GetAddressOf()));
+	}
+
+	void render_and_present(std::uint32_t frame) override
+	{
+		const std::uint32_t bg = pack(static_cast<std::uint8_t>((frame * 2) % 256),
+		                              static_cast<std::uint8_t>((frame * 3) % 256),
+		                              static_cast<std::uint8_t>((frame * 5) % 256));
+		const std::uint32_t whitepx = pack(255, 255, 255);
+		std::uint8_t fr = 0, fg = 0, fb = 0;
+		frame_to_rgb(frame, fr, fg, fb);
+		const std::uint32_t code = pack(fr, fg, fb);
+
+		const std::uint32_t span = width_ > 24 ? width_ - 24 : 1;
+		const std::uint32_t bx = (frame * 4) % span; // moving vertical bar
+		for (std::uint32_t y = 0; y < height_; ++y)
+		{
+			std::uint32_t* row = px_.data() + static_cast<std::size_t>(y) * width_;
+			for (std::uint32_t x = 0; x < width_; ++x)
+			{
+				std::uint32_t c = bg;
+				if (x >= bx && x < bx + 24)
+				{
+					c = whitepx;
+				}
+				if (x < kFrameBlock && y < kFrameBlock)
+				{
+					c = code; // top-left frame-counter block
+				}
+				row[x] = c;
+			}
+		}
+
+		device_->UpdateSubresource(scratch_.Get(), 0, nullptr, px_.data(), static_cast<UINT>(width_ * 4), 0);
+		ComPtr<ID3D10Texture2D> back;
+		if (SUCCEEDED(swap_->GetBuffer(0, IID_PPV_ARGS(back.GetAddressOf()))))
+		{
+			device_->CopyResource(back.Get(), scratch_.Get());
+		}
+		swap_->Present(1, 0); // vsync -> a game-like cadence
+	}
+
+	[[nodiscard]] const char* name() const override
+	{
+		return "dx10";
+	}
+
+private:
+	std::uint32_t width_ = 0;
+	std::uint32_t height_ = 0;
+	std::vector<std::uint32_t> px_;
+	ComPtr<ID3D10Device> device_;
+	ComPtr<IDXGISwapChain> swap_;
+	ComPtr<ID3D10Texture2D> scratch_;
+};
+} // namespace
+
+std::unique_ptr<RenderBackend> create_dx10_backend()
+{
+	return std::make_unique<Dx10Backend>();
+}
+
+} // namespace coop::mock