Function execute_stream 

pub(crate) fn execute_stream(
    site: StreamSite,
    draft: DraftVersion,
    action: StreamAction,
    report: &Reporter<'_>,
) -> Outcome

Execute one StreamAction, at Site::StreamOpen or Site::StreamHeader.

A separate entry point rather than a variant of Target, so that the two sites whose hook methods return StreamAction cannot be handed an Action and Support::NotAttemptable stays unreachable at runtime — a claim made falsifiable by observing zero events at those 30-odd site × action cells.