feat(primitives_gallery): demos para surface_plot_3d + scatter_3d

- demos_3d.cpp con dos demos:
  * demo_surface_plot_3d: malla 64x64 de A*sin(fx*x)*cos(fy*y) con
    sliders fx/fy/amp en tiempo real.
  * demo_scatter_3d: 3 clusters gaussianos (N=500) coloreados por
    cluster, semilla fija para reproducibilidad.
- demos.h: declara las dos demos en la seccion Viz.
- main.cpp: dos entradas nuevas en k_demos[] (Viz, tras heatmap /
  table_view).
- CMakeLists.txt: anade demos_3d.cpp + surface_plot_3d.cpp +
  scatter_3d.cpp al target.

Issue 0028.

cpp/apps/primitives_gallery/CMakeLists.txt

@@ -5,6 +5,7 @@ add_imgui_app(primitives_gallery
     demos_viz.cpp
     demos_graph.cpp
     demos_gfx.cpp
+    demos_3d.cpp
     demos_text_editor.cpp
     demos_gl_texture.cpp
     demos_extras.cpp
@@ -39,6 +40,9 @@ add_imgui_app(primitives_gallery
     ${CMAKE_SOURCE_DIR}/functions/viz/gauge.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/heatmap.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/table_view.cpp
+    # 3D viz primitives (issue 0028, ImPlot3D)
+    ${CMAKE_SOURCE_DIR}/functions/viz/surface_plot_3d.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/scatter_3d.cpp
     # Graph stack (instanced GPU + Barnes-Hut + spatial hash)
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_types.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/graph_renderer.cpp