feat(primitives_gallery): añadir --capture <dir> mode (offscreen render + glReadPixels)

Modo de captura que renderiza cada demo de la gallery en una ventana GLFW
invisible (GLFW_VISIBLE=GLFW_FALSE) y guarda PNG por demo via stb_image_write.

- capture.{h,cpp}: API gallery::run_capture(cfg, items) — warmup_frames,
  glReadPixels(GL_RGBA), flip vertical, stbi_write_png.
- main.cpp: parsea --capture <dir> antes de fn::run_app y delega a capture.cpp.
- vendor: stb_image_write.h v1.16 (mismo commit que stb_image.h).

Funciona en WSL con LIBGL_ALWAYS_SOFTWARE=1 (Mesa/llvmpipe). Si el entorno
no tiene contexto GL, el binario sale con rc!=0 sin generar PNGs.

Issue 0048.

cpp/apps/primitives_gallery/main.cpp

@@ -17,10 +17,12 @@
 
 #include "demos.h"
 #include "demo.h"
+#include "capture.h"
 
 #include <cstdio>
 #include <cstring>
 #include <string>
+#include <sys/stat.h>
 #include <vector>
 
 struct DemoEntry {
@@ -161,7 +163,35 @@ static void render() {
     fn_ui::toast_render();
 }
 
-int main(int /*argc*/, char** /*argv*/) {
+int main(int argc, char** argv) {
+    // Capture mode: `primitives_gallery --capture <output_dir>` corre cada
+    // demo en una ventana GLFW invisible y guarda PNG por demo. Para CI/golden.
+    for (int i = 1; i < argc; i++) {
+        if (std::strcmp(argv[i], "--capture") == 0) {
+            if (i + 1 >= argc) {
+                std::fprintf(stderr, "--capture requires an output dir argument\n");
+                return 2;
+            }
+            const char* out_dir = argv[i + 1];
+            // Best-effort mkdir (idempotente).
+            mkdir(out_dir, 0755);
+
+            std::vector<gallery::CaptureItem> items;
+            items.reserve(k_demo_count);
+            for (int j = 0; j < k_demo_count; j++) {
+                items.push_back({k_demos[j].id, k_demos[j].fn});
+            }
+
+            gallery::CaptureConfig cfg;
+            cfg.output_dir   = out_dir;
+            cfg.warmup_frames = 3;
+            cfg.capture_w    = 800;
+            cfg.capture_h    = 600;
+            const bool ok = gallery::run_capture(cfg, items);
+            return ok ? 0 : 1;
+        }
+    }
+
     return fn::run_app(
         {.title          = "fn_registry · Primitives Gallery",
          .width          = 1400,