feat(primitives_gallery): demo de mesh_viewer (cubo procedural + .obj loader)

Genera cubo procedural in-line (mesh_obj_parse de string), permite
cargar .obj desde un text input absoluto. Botones: Reload cube,
Wireframe toggle, Load .obj. Status line con tris count y
instrucciones (drag to orbit, wheel to zoom).

issue 0029

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_mesh.cpp
     # text_editor + file_watcher (issue 0025)
     ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp
@@ -55,6 +56,11 @@ add_imgui_app(primitives_gallery
     # gl_texture_load (issue 0026) + stb_image
     ${CMAKE_SOURCE_DIR}/functions/gfx/gl_texture_load.cpp
     ${CMAKE_SOURCE_DIR}/vendor/stb/stb_image_impl.cpp
+    # mesh_viewer stack (issue 0029)
+    ${CMAKE_SOURCE_DIR}/functions/gfx/mesh_obj_load.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/mesh_gpu.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/orbit_camera.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/mesh_viewer.cpp
 )
 target_include_directories(primitives_gallery PRIVATE
     ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit

cpp/apps/primitives_gallery/demos.h

@@ -34,6 +34,7 @@ void demo_candlestick();
 void demo_gauge();
 void demo_heatmap();
 void demo_table_view();
+void demo_mesh_viewer();      // issue 0029
 
 // --- Gfx ---
 void demo_shader_canvas();

cpp/apps/primitives_gallery/demos_mesh.cpp

@@ -0,0 +1,108 @@
+// Demo del primitivo viz/mesh_viewer.
+// Genera un cubo procedural in-line, lo sube al GPU, y permite cargar un
+// .obj desde un path ingresado en un text input.
+
+#include "demos.h"
+#include "demo.h"
+
+#include "viz/mesh_viewer.h"
+#include "gfx/mesh_obj_load.h"
+#include "gfx/mesh_gpu.h"
+#include "core/orbit_camera.h"
+
+#include <imgui.h>
+#include <cstring>
+#include <string>
+
+namespace gallery {
+
+namespace {
+
+const char* kCubeObj =
+    "v -1 -1 -1\nv 1 -1 -1\nv 1 1 -1\nv -1 1 -1\n"
+    "v -1 -1 1\nv 1 -1 1\nv 1 1 1\nv -1 1 1\n"
+    "f 4 3 2 1\n"   // back   (-Z) — winding for outward normal
+    "f 5 6 7 8\n"   // front  (+Z)
+    "f 1 2 6 5\n"   // bottom (-Y)
+    "f 8 7 3 4\n"   // top    (+Y)
+    "f 5 8 4 1\n"   // left   (-X)
+    "f 2 3 7 6\n";  // right  (+X)
+
+struct State {
+    fn::gfx::MeshGpu        mesh{};
+    fn::core::OrbitCamera   cam{};
+    char                    path[512] = "";
+    std::string             status;
+    bool                    wireframe = false;
+    bool                    initialized = false;
+};
+
+State& state() {
+    static State s;
+    return s;
+}
+
+void load_cube() {
+    auto& s = state();
+    if (s.mesh.ok()) fn::gfx::mesh_gpu_destroy(s.mesh);
+    auto cpu = fn::gfx::mesh_obj_parse(kCubeObj, std::strlen(kCubeObj));
+    s.mesh = fn::gfx::mesh_gpu_upload(cpu);
+    s.status = s.mesh.ok()
+        ? ("loaded cube: " + std::to_string(s.mesh.index_count / 3) + " tris")
+        : "cube upload failed";
+}
+
+void load_from_path() {
+    auto& s = state();
+    if (!s.path[0]) { s.status = "path is empty"; return; }
+    auto cpu = fn::gfx::mesh_obj_load(s.path);
+    if (cpu.positions.empty()) { s.status = "parse/read failed"; return; }
+    if (s.mesh.ok()) fn::gfx::mesh_gpu_destroy(s.mesh);
+    s.mesh = fn::gfx::mesh_gpu_upload(cpu);
+    s.status = s.mesh.ok()
+        ? ("loaded: " + std::to_string(s.mesh.index_count / 3) + " tris")
+        : "upload failed";
+}
+
+} // namespace
+
+void demo_mesh_viewer() {
+    demo_header("mesh_viewer", "v1.0.0",
+        "Visualizador 3D para inspeccion de geometria. Composicion de "
+        "mesh_obj_load (parser .obj puro) + mesh_gpu (upload VAO/VBO/EBO) + "
+        "orbit_camera (drag/wheel) + mesh_viewer (FBO + ImGui::Image + Lambert).");
+
+    auto& s = state();
+    if (!s.initialized) {
+        load_cube();
+        s.initialized = true;
+    }
+
+    // Controls row.
+    if (ImGui::Button("Reload cube")) load_cube();
+    ImGui::SameLine();
+    ImGui::Checkbox("Wireframe", &s.wireframe);
+    ImGui::SameLine();
+    ImGui::TextDisabled("|");
+    ImGui::SameLine();
+    ImGui::SetNextItemWidth(360);
+    ImGui::InputTextWithHint("##obj_path", "absolute path to .obj", s.path, sizeof(s.path));
+    ImGui::SameLine();
+    if (ImGui::Button("Load .obj")) load_from_path();
+
+    ImGui::TextDisabled("status: %s | tris: %d | drag to orbit, wheel to zoom",
+                        s.status.c_str(),
+                        s.mesh.ok() ? s.mesh.index_count / 3 : 0);
+
+    ImGui::Separator();
+
+    fn::viz::MeshViewerConfig cfg{};
+    cfg.mesh      = &s.mesh;
+    cfg.cam       = &s.cam;
+    cfg.size      = ImVec2(-1.0f, 480.0f);
+    cfg.color     = IM_COL32(160, 200, 255, 255);
+    cfg.wireframe = s.wireframe;
+    fn::viz::mesh_viewer("##gallery_mesh_viewer", cfg);
+}
+
+} // namespace gallery

cpp/apps/primitives_gallery/main.cpp

@@ -61,6 +61,7 @@ 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},
+    {"mesh_viewer",     "mesh_viewer",     "Viz",  &gallery::demo_mesh_viewer},
     // 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