Initial commit: scene-actor-extraction pipeline

Source (KPN++ pipeline nodes, ArcFace embedders, SCRFD/YuNet detectors,
gallery builder), build scripts, and eval artifacts.

- external/KPN as a git submodule (gitea.tourolle.paris/dtourolle/KPN)
- ONNX models tracked via Git LFS (models/*.onnx)
- generated outputs, TensorRT engines, reference repos, and media ignored

src/gallery/gallery_store.hpp

@@ -0,0 +1,20 @@
+#pragma once
+#include "types.hpp"
+#include <string>
+
+// Load/save the actor gallery from/to a JSON file.
+//
+// JSON format:
+// {
+//   "actors": [
+//     {
+//       "imdb_id":      "nm0000093",
+//       "name":         "Brad Pitt",
+//       "source_images": ["img1.jpg", "img2.jpg"],
+//       "embeddings":   [[0.012, -0.034, ...], ...]   // one 512-float array per image
+//     }
+//   ]
+// }
+
+ActorGallery load_gallery(const std::string& path);
+void         save_gallery(const std::string& path, const ActorGallery& gallery);