Merges the terminal states of fan-out branches at a join.

A join carrying an explicit :reduce reducer delegates the whole list of branch states to that function. Otherwise the branch states fold left-to-right in branch-index order: a key declared on a Raxol.Workflow.Channel is combined via the channel's :with reducer, and any other key is last-write-wins by branch order.

Pure: given the same branch states, join, and channels it always returns the same merged state and touches nothing outside its arguments.