Constant MIN_EMBED_PASSAGE_FAN_OUT 

pub const MIN_EMBED_PASSAGE_FAN_OUT: usize = 1;

Lowest REST fan-out width the batched passage embedder will use (v1.2.8, plan step 6).

One means serial: a single batch is one REST call, so the JoinSet would only add latency. Zero would mean “no worker”, which is not a narrower fan-out but an absent one, so the floor is a refusal and not a preference.