add update_vocabulary dora input for runtime label changes

The open_vocab engine now accepts an update_vocabulary input event
carrying a UTF-8 comma-separated label string. Labels are re-encoded
immediately via the CLIP text encoder; the next tick uses the new
vocabulary. ZedNative ignores the input via a no-op base class method.

src/engines/open_vocab.cc

@@ -576,6 +576,11 @@ void OpenVocab::detect(const cv::Mat& image_bgr) {
     }
 }
 
+void OpenVocab::set_labels(const std::string& labels) {
+    config_.labels = labels;
+    encode_labels(labels);
+}
+
 const std::vector<DetectedObject>& OpenVocab::objects() const {
     return detected_;
 }