Function refund_at 

fn refund_at(
    table: &Mutex<Table>,
    key: &str,
    hold: Hold,
    limit: Limit,
    now: Instant,
) -> Result

Give a held reservation’s token back, never past the burst.

A bucket refunded back to full carries no information and is removed, so ordinary sign-ins do not fill the table. An untracked reservation took nothing and gets nothing back.