add open-vocab YOLO-World engine with CLIP text encoder

- Add OpenVocab engine: YOLO-World ONNX detection with runtime
  vocabulary via CLIP text embeddings (two-input model support)
- Add header-only C++ CLIP BPE tokenizer (no external deps)
- Add detect(cv::Mat) overload to OpenVocab for camera-free testing
- Add standalone test binary (test_open_vocab) using test/assets images
- Add export scripts for YOLO-World and CLIP text encoder (uv/PEP 723)
- Add installation scripts for Linux x64, ARM (Jetson), and Windows
- Update CMakeLists to use find_package(onnxruntime) instead of FetchContent
- Update README and AGENTS with env vars, setup workflow, and test usage

src/engines/open_vocab.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include "engines/engine.h"
+
+#include <opencv2/core.hpp>
+#include <memory>
+#include <string>
+
+namespace engines {
+
+struct OpenVocabConfig {
+    std::string model_path;              // ZED_ONNX_MODEL_PATH (required)
+    float confidence_threshold = 0.25f;  // ZED_ONNX_CONFIDENCE
+    float nms_threshold = 0.45f;         // ZED_ONNX_NMS_THRESHOLD
+    int input_size = 640;                // ZED_ONNX_INPUT_SIZE
+    std::string labels;                  // ZED_ONNX_LABELS (comma-separated)
+
+    // Open-vocabulary text encoder (required when model has two inputs)
+    std::string text_encoder_path;       // ZED_ONNX_TEXT_ENCODER_PATH
+    std::string clip_vocab_path;         // ZED_ONNX_CLIP_VOCAB
+    std::string clip_merges_path;        // ZED_ONNX_CLIP_MERGES
+
+    static OpenVocabConfig from_env();
+};
+
+class OpenVocab : public Engine {
+public:
+    explicit OpenVocab(const OpenVocabConfig& config);
+    ~OpenVocab() override;
+
+    void enable(sl::Camera& camera) override;
+    void detect(sl::Camera& camera) override;
+    void detect(const cv::Mat& image_bgr);  // image-only; no 3D fusion
+    const std::vector<DetectedObject>& objects() const override;
+    std::string label_names() const override;
+
+private:
+    OpenVocabConfig config_;
+    struct Impl;
+    std::unique_ptr<Impl> impl_;
+    std::vector<DetectedObject> detected_;
+
+    void encode_labels(const std::string& labels_str);
+};
+
+} // namespace engines