Canonical string form of a stable tab id: t1, t2, … The t prefix
disambiguates stable ids from positional indices (which the CLI no longer
accepts) and matches the @e<N> convention used for element refs.
Labels must look like identifiers: start with a letter, contain only
letters/digits/dashes/underscores. This keeps them distinguishable from
t<N> ids at a glance and safe to pass through shells without quoting.