feat(primitives_gallery): demos para los 5 charts cientificos (issue 0034)

Anade 5 entradas a la gallery (treemap, sankey, chord, contour, voronoi)
con datos sinteticos coherentes:
- treemap: 6 items 'gastos por categoria'
- sankey: 8 nodos clientes -> productos -> categorias (DAG)
- chord: matriz 6x6 simetrica de flujos entre paises
- contour: mezcla de 2 gaussianas 32x32 + 5 niveles
- voronoi: 30 seeds aleatorias + colores aleatorios

Wire-up additions only (demos.h, main.cpp k_demos[], CMakeLists.txt).

cpp/apps/primitives_gallery/main.cpp

@@ -61,6 +61,11 @@ 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},
+    {"treemap",         "treemap",         "Viz",  &gallery::demo_treemap},
+    {"sankey",          "sankey",          "Viz",  &gallery::demo_sankey},
+    {"chord",           "chord",           "Viz",  &gallery::demo_chord},
+    {"contour",         "contour",         "Viz",  &gallery::demo_contour},
+    {"voronoi",         "voronoi",         "Viz",  &gallery::demo_voronoi},
     // 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