EffectOutcome

An Effect and what running it produced.

The pairing lives here rather than as a field on EffectResult because EffectSuccess is an object — there is one of it, shared by every effect that succeeded, so it has nowhere to put "which effect". Pairing at the boundary also leaves every existing EffectResult implementation, transport ones included, exactly as it was.

Why it exists at all: StateDispatcher.onEffectResults used to receive a bare list of results, and nothing connected it to the list of effects that produced them — not an index, not an id. A dispatcher that sent two messages and needed the id of the first had to assume the two lists line up positionally. That assumption is correct until somebody adds an effect to the transition, and then it is silently wrong: the dispatcher edits the wrong message, and no test that asserts on a single effect can see it.