Macro sql_expr 

macro_rules! sql_expr {
    ($template:expr) => { ... };
    ($template:expr, $($param:tt),*) => { ... };
}

Create a SQL expression with serde_json::Value as the value type.

This is the untyped variant — parameters are plain JSON values without type markers. Useful for statement builders (Step 3) where types are inferred from context.

• sql_expr!("template") — no parameters
• sql_expr!("template", scalar) — scalar becomes ExpressiveEnum::Scalar(scalar.into())
• sql_expr!("template", (expr)) — expr becomes ExpressiveEnum::Nested(expr.expr())