Function classify_topic_stub 

pub fn classify_topic_stub(message: &str) -> Topic

Classify the topic of a message (LLM-based, Phase 4 implementation).

Currently returns a conservative stub that classifies common topics without LLM. Phase 4 replaces this with actual LLM integration.

The classifier_fn is injected so the actual LLM call can be wired in at the Orchestrator level without this module knowing about providers.