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/demos.h

@@ -34,6 +34,11 @@ void demo_candlestick();
 void demo_gauge();
 void demo_heatmap();
 void demo_table_view();
+void demo_treemap();          // issue 0034
+void demo_sankey();            // issue 0034
+void demo_chord();             // issue 0034
+void demo_contour();           // issue 0034
+void demo_voronoi();           // issue 0034
 
 // --- Gfx ---
 void demo_shader_canvas();