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/engine.h

@@ -20,6 +20,7 @@ public:
 
     virtual void enable(sl::Camera& camera) = 0;
     virtual void detect(sl::Camera& camera) = 0;
+    virtual void set_labels(const std::string& /*labels*/) {}
     virtual const std::vector<DetectedObject>& objects() const = 0;
     virtual std::string label_names() const = 0;
 };