feat: add C++ ImGui functions for core UI and visualization

Funciones C++/ImGui para dashboards (grid, panel, docking, sidebar, tabs),
visualizaciones (candlestick, gauge, histogram, pie, sparkline, heatmap,
scatter, line, bar, surface3d, kpi, table), grafos (force layout, renderer,
viewport, spatial hash, types) y utilidades (time series buffer, tracy zones,
memory/fps overlay, plot theme).

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

cpp/functions/viz/surface_plot_3d.h

@@ -0,0 +1,8 @@
+#pragma once
+
+// [STUB] Renders a 3D surface plot using ImPlot3D.
+// Requires ImPlot3D to be vendored in cpp/vendor/implot3d/.
+// Until then, displays a placeholder message inside an ImGui group.
+// Call within an ImGui frame (inside fn::run_app render callback).
+void surface_plot_3d(const char* title, const float* values, int rows, int cols,
+                     float z_min, float z_max);