Add the renderer process

The renderer becomes two programs: the adapter (t5deRenderEngine.dll, 32-bit, in IMVU's process) and a 64-bit renderer process (t5de-renderer.exe) drawing with Diligent Engine, fetched with FetchContent and pinned. For now a spike: the adapter starts the renderer process for each engine and signals each frame through a shared memory channel, and the renderer process draws a box into a child window in the client's 3D view.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

adapter/renderer_process.hpp

@@ -0,0 +1,35 @@
+#pragma once
+// The renderer process (t5de-renderer.exe, 64-bit) an engine draws through: started with the engine, told about
+// each frame through a shared memory channel (include/t5de/protocol/channel.hpp), and told to exit with it.
+
+#include <windows.h>
+
+#include <cstdint>
+#include <memory>
+
+#include "t5de/protocol/channel.hpp"
+
+namespace t5de {
+
+class RendererProcess {
+public:
+    // Starts t5de-renderer.exe from this DLL's folder, drawing into window; null if it can't.
+    static std::unique_ptr<RendererProcess> start(HWND window);
+
+    RendererProcess(const RendererProcess&) = delete;
+    RendererProcess& operator=(const RendererProcess&) = delete;
+    ~RendererProcess();
+
+    // The client drew a frame.
+    void frameDrawn();
+
+private:
+    RendererProcess() = default;
+
+    HANDLE mapping_ = nullptr;
+    HANDLE frameEvent_ = nullptr;
+    HANDLE process_ = nullptr;
+    protocol::Channel* channel_ = nullptr;
+};
+
+} // namespace t5de