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/functions/viz/scatter_plot.h

@@ -0,0 +1,6 @@
+#pragma once
+
+// Renders a scatter plot using ImPlot.
+// Call within an ImGui frame.
+void scatter_plot(const char* title, const float* xs, const float* ys, int count);
+void scatter_plot(const char* title, const double* xs, const double* ys, int count);