Function is_bool_stringify 

pub fn is_bool_stringify(node: &AIRNode) -> bool

True when an expression node is a Bool value that must stringify to the canonical lowercase "true" / "false" (§3.5) — the checker stamped it with bock_types::checker::BOOL_STRINGIFY_META_KEY because it appears as an ${expr} interpolation part of Bool type.

Only the Python backend consults this (its f"{b}" prints True/False); JS/TS template literals and Rust/Go formatting already print lowercase.