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

cpp/apps/primitives_gallery/demos.h

@@ -34,6 +34,8 @@ void demo_candlestick();
 void demo_gauge();
 void demo_heatmap();
 void demo_table_view();
+void demo_surface_plot_3d();   // issue 0028, ImPlot3D
+void demo_scatter_3d();        // issue 0028, ImPlot3D
 
 // --- Gfx ---
 void demo_shader_canvas();

cpp/apps/primitives_gallery/demos_3d.cpp

@@ -0,0 +1,100 @@
+// demos_3d — demos para los primitivos viz/* basados en ImPlot3D.
+// Issue 0028: surface_plot_3d real + scatter_3d.
+
+#include "demos.h"
+#include "demo.h"
+
+#include "viz/surface_plot_3d.h"
+#include "viz/scatter_3d.h"
+
+#include <imgui.h>
+#include <cmath>
+#include <random>
+#include <vector>
+
+namespace gallery {
+
+// ---------------------------------------------------------------------------
+// surface_plot_3d
+// ---------------------------------------------------------------------------
+
+void demo_surface_plot_3d() {
+    demo_header("surface_plot_3d", "v2.0.0",
+        "Superficie 3D ImPlot3D (z = A * sin(fx*x) * cos(fy*y)) con sliders para "
+        "ajustar las frecuencias en tiempo real. Drag para orbitar, wheel para zoom.");
+
+    section("Malla 64x64 — sin(fx*x) * cos(fy*y)");
+
+    static float fx = 0.20f;
+    static float fy = 0.20f;
+    static float amp = 1.0f;
+
+    ImGui::SliderFloat("fx", &fx, 0.05f, 1.0f, "%.2f");
+    ImGui::SliderFloat("fy", &fy, 0.05f, 1.0f, "%.2f");
+    ImGui::SliderFloat("amplitud", &amp, 0.1f, 3.0f, "%.2f");
+
+    constexpr int N = 64;
+    static std::vector<float> z(N * N);
+    for (int j = 0; j < N; ++j) {
+        for (int i = 0; i < N; ++i) {
+            z[j * N + i] = amp * std::sin(fx * float(i)) * std::cos(fy * float(j));
+        }
+    }
+
+    fn::SurfacePlot3DConfig cfg{};
+    cfg.z = z.data();
+    cfg.nx = N; cfg.ny = N;
+    cfg.x_min = 0.f; cfg.x_max = float(N);
+    cfg.y_min = 0.f; cfg.y_max = float(N);
+    cfg.size = ImVec2(-1.f, 420.f);
+    fn::surface_plot_3d("##gallery_surface", cfg);
+}
+
+// ---------------------------------------------------------------------------
+// scatter_3d
+// ---------------------------------------------------------------------------
+
+void demo_scatter_3d() {
+    demo_header("scatter_3d", "v1.0.0",
+        "Scatter 3D ImPlot3D con color por punto. 3 clusters gaussianos sinteticos "
+        "(N=500) para simular una visualizacion tipica de PCA / clustering.");
+
+    section("3 clusters gaussianos (500 puntos)");
+
+    constexpr int N = 500;
+    static std::vector<float> xs(N), ys(N), zs(N);
+    static std::vector<ImU32> colors(N);
+    static bool initialized = false;
+
+    if (!initialized) {
+        std::mt19937 rng(42);
+        std::normal_distribution<float> g(0.f, 0.4f);
+        const ImU32 palette[3] = {
+            IM_COL32(255,  99,  71, 255),  // tomate
+            IM_COL32( 65, 170, 255, 255),  // azul
+            IM_COL32(120, 220, 120, 255),  // verde
+        };
+        const float cx[3] = {-1.5f, 1.5f, 0.f};
+        const float cy[3] = { 0.f,  0.f,  2.0f};
+        const float cz[3] = { 0.f,  1.0f,-1.0f};
+        for (int i = 0; i < N; ++i) {
+            int c = i % 3;
+            xs[i] = cx[c] + g(rng);
+            ys[i] = cy[c] + g(rng);
+            zs[i] = cz[c] + g(rng);
+            colors[i] = palette[c];
+        }
+        initialized = true;
+    }
+
+    fn::Scatter3DConfig cfg{};
+    cfg.xs = xs.data();
+    cfg.ys = ys.data();
+    cfg.zs = zs.data();
+    cfg.colors = colors.data();
+    cfg.n = N;
+    cfg.size = ImVec2(-1.f, 420.f);
+    fn::scatter_3d("##gallery_clusters", cfg);
+}
+
+} // namespace gallery

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