Function redock_as_tab 

async fn redock_as_tab(
    state: &AppState,
    target_tab_id: &str,
    block_id: &str,
    target_block_id: &str,
) -> Result<(), String>

Place a just-redocked block as a TAB of the pane holding target_block_id, active: the reducer’s own tree first (LayoutStackPush, which also checks the block now lives in this tab), then a queued stackpush so the window showing that tab adds the pill. Same backend-then-frontend shape as pane.moveTab.