Function handle_create_block_in_stack 

pub(super) fn handle_create_block_in_stack(
    state: &mut State,
    tab_id: String,
    target_block_id: String,
    meta: Value,
    activate: bool,
) -> Vec<Event>

Create a block and place it in a pane’s tab stack in one step. Placement is validated FIRST: if no pane holds target_block_id, nothing is created — a block never exists without a pane.