Function handle_reorder_tab 

pub(super) fn handle_reorder_tab(
    state: &mut State,
    workspace_id: String,
    tab_id: String,
    new_index: u32,
) -> Vec<Event>

Phase E.2c.3b — reorder a tab within its workspace’s tab_ids. new_index is clamped to tab_ids.len(). No-op if the tab is already at that position. Errors (non-fatal) if the workspace doesn’t exist or the tab isn’t in its tab list.