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/CMakeLists.txt

@@ -8,6 +8,7 @@ add_imgui_app(primitives_gallery
     demos_text_editor.cpp
     demos_gl_texture.cpp
     demos_extras.cpp
+    demos_scientific.cpp
     # text_editor + file_watcher (issue 0025)
     ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp
@@ -39,6 +40,12 @@ 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
+    # Scientific viz (issue 0034)
+    ${CMAKE_SOURCE_DIR}/functions/viz/treemap.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/sankey.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/chord.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/contour.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/voronoi.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