Type Alias CustomOp 

pub type CustomOp = dyn Fn(&[Value]) -> Result<Value, Box<dyn Error + Send + Sync>> + Send + Sync;

Signature for a custom operator handler.

Receives a slice of already-evaluated argument values (not raw AST nodes). Returns a Value that the evaluator inserts back into the expression tree, or an error that surfaces as GuardError::CustomOpFailed.