Function handle_set_focused_node 

pub(super) fn handle_set_focused_node(
    state: &mut State,
    tab_id: String,
    node_id: String,
) -> Vec<Event>

Phase E.4 (Option A) — set the focused layout-node id on a tab. Errors (non-fatal) if the tab is unknown to the reducer; no-op short-circuit when the value is already current. Empty node_id clears the field. Bumps the version only on real changes so a burst of identical sets doesn’t churn the event stream.