Function last 

pub fn last(core: &Core, source: NodeId) -> FlowRegistration

last(source) — buffers the latest DATA; emits Data(latest) then Complete on upstream COMPLETE. On empty stream (no DATA arrived), emits only Complete — subscribers see [Start, Complete]. For a fallback value on empty streams, use last_with_default.

Opts out of Lock 2.B auto-cascade so it can intercept upstream COMPLETE (same pattern as reduce).