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.cpp

@@ -0,0 +1,58 @@
+#include "gallery_store.hpp"
+
+#include <nlohmann/json.hpp>
+#include <fstream>
+#include <stdexcept>
+
+using json = nlohmann::json;
+
+ActorGallery load_gallery(const std::string& path) {
+    std::ifstream f(path);
+    if (!f.is_open())
+        throw std::runtime_error("load_gallery: cannot open " + path);
+
+    json j;
+    f >> j;
+
+    ActorGallery gallery;
+    for (const auto& ja : j.at("actors")) {
+        ActorGallery::Actor actor;
+        actor.imdb_id = ja.at("imdb_id").get<std::string>();
+        actor.name    = ja.at("name").get<std::string>();
+
+        if (ja.contains("source_images"))
+            actor.source_images = ja.at("source_images").get<std::vector<std::string>>();
+
+        for (const auto& je : ja.at("embeddings")) {
+            Embedding emb = je.get<Embedding>();
+            actor.embeddings.push_back(emb);
+        }
+
+        gallery.actors.push_back(std::move(actor));
+    }
+    return gallery;
+}
+
+void save_gallery(const std::string& path, const ActorGallery& gallery) {
+    json j;
+    j["actors"] = json::array();
+
+    for (const auto& actor : gallery.actors) {
+        json ja;
+        ja["imdb_id"]      = actor.imdb_id;
+        ja["name"]         = actor.name;
+        ja["source_images"] = actor.source_images;
+
+        ja["embeddings"] = json::array();
+        for (const auto& emb : actor.embeddings) {
+            ja["embeddings"].push_back(
+                std::vector<float>(emb.begin(), emb.end()));
+        }
+        j["actors"].push_back(std::move(ja));
+    }
+
+    std::ofstream f(path);
+    if (!f.is_open())
+        throw std::runtime_error("save_gallery: cannot write " + path);
+    f << j.dump(2) << "\n";
+}