feat(primitives_gallery): demo de gl_texture_load + asset PNG

Añade demos_gl_texture.cpp (carga assets/sample.png con la nueva
funcion gl_texture_load_cpp_gfx, muestra w/h/channels y sliders de
tint RGB + zoom UV via ImGui::ImageWithBg) y un PNG damero 256x256
generado para la demo.

Tambien registra apps/primitives_gallery como subdirectorio en
cpp/CMakeLists.txt (la app vive como WIP en master; aqui solo se
añade el hook de build).

Para integrarse, demos.h, main.cpp y CMakeLists.txt de la gallery
deben anadir respectivamente la declaracion gallery::demo_gl_texture(),
la entrada {"gl_texture", "gl_texture_load", "Gfx", &gallery::demo_gl_texture}
en k_demos[], y demos_gl_texture.cpp + cpp/functions/gfx/gl_texture_load.cpp +
cpp/vendor/stb/stb_image_impl.cpp + include de cpp/vendor/stb a sus sources.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/CMakeLists.txt

@@ -115,6 +115,11 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/shaders_lab/CMakeLists.txt)
     add_subdirectory(apps/shaders_lab)
 endif()
 
+# --- Primitives Gallery ---
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/apps/primitives_gallery/CMakeLists.txt)
+    add_subdirectory(apps/primitives_gallery)
+endif()
+
 # --- Registry Dashboard (lives in projects/fn_monitoring/apps/) ---
 set(_DASH_DIR ${CMAKE_SOURCE_DIR}/../projects/fn_monitoring/apps/registry_dashboard)
 if(EXISTS ${_DASH_DIR}/CMakeLists.txt)

cpp/apps/primitives_gallery/demos_gl_texture.cpp

@@ -0,0 +1,127 @@
+// Demo de gl_texture_load (cpp/functions/gfx/gl_texture_load.{h,cpp}).
+// Carga assets/sample.png y lo muestra con ImGui::Image. Sliders para tint
+// RGB que se aplican como modulacion (ImGui::Image acepta tint_col).
+//
+// Limitacion: el "zoom UV" se simula moviendo uv0/uv1 (que ImGui::Image acepta
+// nativamente). Asi evitamos compilar un shader custom adicional para la demo.
+
+#include "demos.h"
+#include "demo.h"
+
+#include "gfx/gl_texture_load.h"
+#include "gfx/gl_loader.h"
+
+#include <imgui.h>
+#include <cstdio>
+#include <cstring>
+
+namespace gallery {
+
+namespace {
+
+struct TextureState {
+    fn::GlTexture tex{};
+    bool          tried_load = false;
+    std::string_view err;
+    char          err_buf[256] = {0};
+    float         tint[3]      = {1.0f, 1.0f, 1.0f};
+    float         zoom          = 1.0f; // 1.0 = sin zoom; >1 hace UV mas pequeno
+};
+
+TextureState& state() {
+    static TextureState s;
+    return s;
+}
+
+// Resuelve un path para el asset. Probamos varios candidatos relativos al cwd
+// del binario (puede lanzarse desde build/ o desde la raiz del repo).
+const char* resolve_sample_path() {
+    static const char* candidates[] = {
+        "assets/sample.png",
+        "apps/primitives_gallery/assets/sample.png",
+        "cpp/apps/primitives_gallery/assets/sample.png",
+        "../cpp/apps/primitives_gallery/assets/sample.png",
+        "../../cpp/apps/primitives_gallery/assets/sample.png",
+        "../../../cpp/apps/primitives_gallery/assets/sample.png",
+        nullptr,
+    };
+    for (int i = 0; candidates[i]; i++) {
+        FILE* f = std::fopen(candidates[i], "rb");
+        if (f) { std::fclose(f); return candidates[i]; }
+    }
+    return candidates[0]; // devolver el primer candidato para que el error sea mas descriptivo
+}
+
+} // namespace
+
+void demo_gl_texture() {
+    demo_header("gl_texture_load", "v1.0.0",
+        "Carga PNG/JPG/HDR desde disco a una textura GL lista para sampler2D. "
+        "Vendorea stb_image (cpp/vendor/stb/). Demo: assets/sample.png "
+        "(damero 256x256), tint RGB modulando ImGui::Image, zoom UV.");
+
+    auto& s = state();
+
+    if (!s.tried_load) {
+        // Asegurar simbolos GL resueltos (Linux no-op, Windows wglGetProcAddress).
+        fn::gfx::gl_loader_init();
+        const char* path = resolve_sample_path();
+        s.tex = fn::gl_texture_load(path, /*flip_y=*/true, /*srgb=*/false);
+        if (!s.tex.ok()) {
+            std::snprintf(s.err_buf, sizeof(s.err_buf),
+                "no se pudo cargar '%s': %s",
+                path, fn::gl_texture_last_error());
+        }
+        s.tried_load = true;
+    }
+
+    if (!s.tex.ok()) {
+        ImGui::TextColored(ImVec4(1.0f, 0.4f, 0.4f, 1.0f), "%s", s.err_buf);
+        ImGui::TextWrapped(
+            "El binario busca el PNG en varios paths relativos al cwd. "
+            "Lanzar desde la raiz del repo o desde cpp/build/ deberia funcionar.");
+        return;
+    }
+
+    section("Texture info");
+    ImGui::Text("size:     %d x %d px", s.tex.w, s.tex.h);
+    ImGui::Text("channels: %d (forzado a RGBA en upload)", s.tex.channels);
+    ImGui::Text("gl_id:    %u", (unsigned)s.tex.id);
+
+    section("Tint + zoom");
+    ImGui::SliderFloat3("tint RGB", s.tint, 0.0f, 2.0f, "%.2f");
+    ImGui::SliderFloat("zoom UV",   &s.zoom, 0.25f, 4.0f, "%.2fx");
+
+    section("Preview");
+
+    // Calcular UVs centradas con zoom: 1.0 = (0,0)-(1,1), 2.0 = (0.25,0.25)-(0.75,0.75)
+    float u_half = 0.5f / (s.zoom > 0.001f ? s.zoom : 0.001f);
+    ImVec2 uv0(0.5f - u_half, 0.5f - u_half);
+    ImVec2 uv1(0.5f + u_half, 0.5f + u_half);
+
+    ImVec4 tint(s.tint[0], s.tint[1], s.tint[2], 1.0f);
+
+    // Conversion GLuint -> ImTextureID. ImGui::Image acepta cualquier id de
+    // textura del backend; en imgui_impl_opengl3 es directamente el GLuint.
+    ImTextureID tid = (ImTextureID)(intptr_t)s.tex.id;
+
+    ImGui::ImageWithBg(tid, ImVec2(384.0f, 384.0f), uv0, uv1,
+                       ImVec4(0, 0, 0, 0), tint);
+
+    code_block(
+        "#include \"gfx/gl_texture_load.h\"\n\n"
+        "auto tex = fn::gl_texture_load(\"assets/sample.png\");\n"
+        "if (!tex.ok()) {\n"
+        "    fprintf(stderr, \"%s\\n\", fn::gl_texture_last_error());\n"
+        "    return 1;\n"
+        "}\n"
+        "// uso en shader:\n"
+        "glUseProgram(prog);\n"
+        "fn::gl_texture_bind_uniform(prog, \"u_tex\", tex, /*unit=*/0);\n"
+        "glDrawArrays(GL_TRIANGLES, 0, 6);\n\n"
+        "// o en ImGui directamente:\n"
+        "ImGui::Image((ImTextureID)(intptr_t)tex.id, ImVec2(w, h));"
+    );
+}
+
+} // namespace gallery