Function classify_statement 

pub fn classify_statement(sql: &str) -> StatementKind

Coarse-classify the first SQL statement in sql after stripping leading whitespace and line/block comments.

First-keyword only: a WITH x AS (DELETE …) SELECT … CTE is classified as ReadOnly even though it mutates. Hyper itself rejects data-modifying CTEs, so this is a defense-in-depth heuristic rather than the only barrier.