Function queue_target_stack_move 

pub(crate) async fn queue_target_stack_move(
    state: &AppState,
    tab_id: &str,
    block_id: &str,
    target_block_id: &str,
    position: &str,
    activate: bool,
) -> Result<(), String>

Tell the frontend that block_id was moved — already, in the reducer — to position relative to target_block_id (Command::LayoutStackMove): a same-pane reorder when both are already in one leaf’s stack, otherwise a cross-pane move. Matches LayoutTreeActionType.StackMove = "stackmove" in frontend/layout/lib/types.ts. position is the same "before"/"after"/"end" string pane.moveTab’s caller supplied — reused verbatim, not re-derived, so the frontend applies the identical operation rather than inferring it from resulting tree state. activate rides on LayoutActionData.focused — the same field queue_target_stack_push above already overloads for this purpose (that call hardcodes true, since CreateBlockInStack always activates; this one carries the caller’s actual activate value since a reorder may or may not). SPEC_PANE_TAB_DRAG_AND_DROP_2026_09_19.md §4.1, Phase 3.