Function apply_layout_cleared 

fn apply_layout_cleared(
    mstore: &Store,
    tab_id: &str,
) -> Result<(), Box<dyn Error>>

Phase E.4.B — apply a LayoutCleared event: wipe the tab’s persisted layout tree and the focus/magnify ids (and the leaforder cache — an empty tree has no leaves). Mirrors reducer::layout::handle_layout_clear. Idempotent: silent no-op when the tab is unknown, the layout row is missing, or it is already clear (so a double-apply — once synchronously from the RPC handler, once from the broadcast — does not churn the version).