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.h

@@ -30,7 +30,8 @@ public:
 
     void enable(sl::Camera& camera) override;
     void detect(sl::Camera& camera) override;
-    void detect(const cv::Mat& image_bgr);  // image-only; no 3D fusion
+    void detect(const cv::Mat& image_bgr);       // image-only; no 3D fusion
+    void set_labels(const std::string& labels) override;
     const std::vector<DetectedObject>& objects() const override;
     std::string label_names() const override;