smolModel property
The resolved smol summarizer model, when one is configured AND
distinct from the main model (a smol equal to the main model is
normalized to null — the AutoCompactor skips the smol→main
fallback for a same-model pair, so the wiring says so up front).
null = the main stream summarizes (main-model fallback as last
resort, gh-1077 fix contract 1).
Implementation
final Model? smolModel;