Function has_stable_string_form 

pub fn has_stable_string_form(data_type: &ConcreteDataType) -> bool

Returns true if a column of data_type renders as a stable string — the requirement for entity id/descriptive/scope columns. The read-time derivation casts them to strings, so a type without a stable string form would fail only when the graph is scanned; DDL validation rejects it up front instead.