M2(DX9): dual-mode DX9 mock-game render backend

Add render_dx09.cpp (selectable as `dx9ex` / `dx9`): renders the animated
pattern with Clear + ColorFill (D3D9's built-in rect fill -- no shaders) and
presents through a real D3D9 / D3D9Ex device. Two modes so the two D3D9 capture
paths each have a matching game. Smoke-verified: both present frames and exit
cleanly. The capture hook + frame-decode test land next.

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

tools/mock_game/render_dx09.cpp

@@ -0,0 +1,118 @@
+// DX9 backend for the mock game. Renders the animated pattern with Clear + ColorFill (D3D9's
+// built-in rect fill -- no shaders) and presents through a real D3D9 device, so the D3D9
+// capture hook sees a genuine present. Two selectable modes: "dx9ex" (Direct3DCreate9Ex) and
+// plain "dx9" (Direct3DCreate9), so the two D3D9 capture paths each have a matching game. The
+// back buffer is the standard A8R8G8B8 (BGRA byte order); the capture side swizzles to RGBA.
+#include "render_backend.hpp"
+
+#include <d3d9.h>
+#include <wrl/client.h>
+
+using Microsoft::WRL::ComPtr;
+
+namespace coop::mock
+{
+namespace
+{
+D3DCOLOR opaque(std::uint8_t r, std::uint8_t g, std::uint8_t b)
+{
+	return D3DCOLOR_ARGB(255, r, g, b);
+}
+
+class Dx9Backend : public RenderBackend
+{
+public:
+	explicit Dx9Backend(bool ex) : ex_(ex)
+	{
+	}
+
+	bool init(HWND hwnd, std::uint32_t width, std::uint32_t height) override
+	{
+		width_ = width;
+		height_ = height;
+
+		D3DPRESENT_PARAMETERS pp = {};
+		pp.BackBufferWidth = width;
+		pp.BackBufferHeight = height;
+		pp.BackBufferFormat = D3DFMT_X8R8G8B8;
+		pp.BackBufferCount = 1;
+		pp.SwapEffect = D3DSWAPEFFECT_DISCARD;
+		pp.hDeviceWindow = hwnd;
+		pp.Windowed = TRUE;
+		pp.PresentationInterval = D3DPRESENT_INTERVAL_ONE; // vsync -> a game-like cadence
+
+		if (ex_)
+		{
+			ComPtr<IDirect3D9Ex> d3d;
+			if (FAILED(Direct3DCreate9Ex(D3D_SDK_VERSION, d3d.GetAddressOf())))
+			{
+				return false;
+			}
+			ComPtr<IDirect3DDevice9Ex> dev;
+			if (FAILED(d3d->CreateDeviceEx(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hwnd,
+			                               D3DCREATE_HARDWARE_VERTEXPROCESSING, &pp, nullptr,
+			                               dev.GetAddressOf())))
+			{
+				return false;
+			}
+			dev_ = dev; // IDirect3DDevice9Ex derives from IDirect3DDevice9
+		}
+		else
+		{
+			ComPtr<IDirect3D9> d3d(Direct3DCreate9(D3D_SDK_VERSION));
+			if (!d3d)
+			{
+				return false;
+			}
+			if (FAILED(d3d->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, hwnd,
+			                             D3DCREATE_HARDWARE_VERTEXPROCESSING, &pp, dev_.GetAddressOf())))
+			{
+				return false;
+			}
+		}
+		return true;
+	}
+
+	void render_and_present(std::uint32_t frame) override
+	{
+		const D3DCOLOR bg = opaque(static_cast<std::uint8_t>((frame * 2) % 256),
+		                           static_cast<std::uint8_t>((frame * 3) % 256),
+		                           static_cast<std::uint8_t>((frame * 5) % 256));
+		dev_->Clear(0, nullptr, D3DCLEAR_TARGET, bg, 1.0f, 0);
+
+		ComPtr<IDirect3DSurface9> back;
+		if (SUCCEEDED(dev_->GetBackBuffer(0, 0, D3DBACKBUFFER_TYPE_MONO, back.GetAddressOf())))
+		{
+			const LONG span = width_ > 24 ? static_cast<LONG>(width_ - 24) : 1;
+			const LONG bx = static_cast<LONG>((frame * 4) % static_cast<std::uint32_t>(span));
+			RECT bar = {bx, 0, bx + 24, static_cast<LONG>(height_)}; // moving vertical bar
+			dev_->ColorFill(back.Get(), &bar, opaque(255, 255, 255));
+
+			std::uint8_t r = 0, g = 0, b = 0;
+			frame_to_rgb(frame, r, g, b);
+			RECT block = {0, 0, static_cast<LONG>(kFrameBlock), static_cast<LONG>(kFrameBlock)};
+			dev_->ColorFill(back.Get(), &block, opaque(r, g, b)); // top-left frame-counter block
+		}
+
+		dev_->Present(nullptr, nullptr, nullptr, nullptr);
+	}
+
+	[[nodiscard]] const char* name() const override
+	{
+		return ex_ ? "dx9ex" : "dx9";
+	}
+
+private:
+	bool ex_;
+	std::uint32_t width_ = 0;
+	std::uint32_t height_ = 0;
+	ComPtr<IDirect3DDevice9> dev_;
+};
+} // namespace
+
+std::unique_ptr<RenderBackend> create_dx9_backend(bool ex)
+{
+	return std::make_unique<Dx9Backend>(ex);
+}
+
+} // namespace coop::mock