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.md

@@ -0,0 +1,38 @@
+---
+name: scatter_plot
+kind: component
+lang: cpp
+domain: viz
+version: "1.0.0"
+purity: pure
+signature: "void scatter_plot(const char* title, const float* xs, const float* ys, int count)"
+description: "Renderiza un grafico de dispersion usando ImPlot dentro de un frame ImGui"
+tags: [implot, chart, visualization, gpu, scatter]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: [implot]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "cpp/functions/viz/scatter_plot.cpp"
+framework: imgui
+params:
+  - name: title
+    desc: "Titulo del grafico scatter"
+  - name: xs
+    desc: "Array de coordenadas X"
+  - name: ys
+    desc: "Array de coordenadas Y"
+  - name: count
+    desc: "Numero de puntos en los arrays xs/ys"
+output: "Renderiza el grafico de dispersion en el frame ImGui actual"
+---
+
+# scatter_plot
+
+Wrapper atomico sobre `ImPlot::PlotScatter`. Renderiza un grafico de dispersion 2D.
+
+Debe llamarse dentro del render callback de `fn::run_app`.