feat: add C++ support with ImGui/ImPlot framework and vendor submodules

Añade soporte C++ al registry: vendor submodules (glfw, imgui, implot, tracy),
sistema de build con CMake y toolchains cross-platform, runner C++ en fn CLI,
parser de tests Google Test, y funciones bash para build Linux/Windows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cpp/framework/app_base.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <functional>
+
+namespace fn {
+
+struct AppConfig {
+    const char* title = "fn_registry";
+    int width = 1280;
+    int height = 720;
+    bool vsync = true;
+    float bg_r = 0.1f;
+    float bg_g = 0.1f;
+    float bg_b = 0.1f;
+};
+
+// Run an ImGui application. The render_fn is called every frame
+// between ImGui::NewFrame() and ImGui::Render().
+// Returns 0 on clean exit, 1 on error.
+int run_app(AppConfig config, std::function<void()> render_fn);
+
+// Convenience: run with default config
+int run_app(std::function<void()> render_fn);
+
+} // namespace fn