Function detect_topic_shift 

pub fn detect_topic_shift(
    current_tag: Option<&str>,
    next_tag_number: u32,
    new_message_embedding: &[f32],
    recent_embeddings: &[Vec<f32>],
) -> TopicSegment

Detect whether a new user message is a continuation of the current topic or a shift to a new one.

Uses embedding cosine similarity against recent assistant responses. Returns the topic tag to assign to this message.

current_tag: the topic tag of the most recent message (e.g. “topic-2”). Pass None for the first message in a session. next_tag_number: the next available topic number (e.g. 3 if “topic-2” is current). new_message_embedding: embedding of the user’s new message. recent_embeddings: embeddings of the most recent LOOKBACK_TURNS messages (from newest to oldest).