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/main.cpp

@@ -61,6 +61,8 @@ static const DemoEntry k_demos[] = {
     {"gauge",           "gauge",           "Viz",  &gallery::demo_gauge},
     {"heatmap",         "heatmap",         "Viz",  &gallery::demo_heatmap},
     {"table_view",      "table_view",      "Viz",  &gallery::demo_table_view},
+    {"surface_plot_3d", "surface_plot_3d", "Viz",  &gallery::demo_surface_plot_3d},
+    {"scatter_3d",      "scatter_3d",      "Viz",  &gallery::demo_scatter_3d},
     // Gfx (shaders_lab core)
     {"shader_canvas",   "shader_canvas",   "Gfx",  &gallery::demo_shader_canvas},
     {"gl_texture",      "gl_texture_load", "Gfx",  &gallery::demo_gl_texture},      // wave 1