Function handle_layout_queue_backend_actions 

pub(super) fn handle_layout_queue_backend_actions(
    state: &mut State,
    tab_id: String,
    actions: Value,
    correlation_id: String,
) -> Vec<Event>

SPEC_864 Phase 4 — append backend actions to a tab’s pendingbackendactions queue. The reducer does NOT model the queue in TabRecord (it is a transient backend→frontend mailbox the frontend drains via REPLACE-clear slices, not layout-tree state), so this arm only validates and passes the payload through; the persist subscriber APPENDs it to LayoutState.pendingbackendactions.

actions must be a non-empty JSON array of LayoutActionData objects (the type lives in agentmux-srv::backend::obj, not the common crate — hence the raw-value carriage).