refactor: replace BinOp string with typed enum in Expr AST

Previously Expr::BinOp(String, ...) accepted any string as an operator.
Invalid operators (e.g. "diagonal") would compile fine and silently
return CellValue::Empty at eval time.

Now BinOp is an enum with variants Add/Sub/Mul/Div/Pow/Eq/Ne/Lt/Gt/Le/Ge.
The parser produces enum variants directly; the evaluator pattern-matches
exhaustively with no fallback branch. An invalid operator is now a
compile error at the call site, and the compiler ensures every variant
is handled in both eval_expr and eval_bool.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/formula/parser.rs

@@ -1,6 +1,6 @@
 use anyhow::{anyhow, Result};
 
-use super::ast::{AggFunc, Expr, Filter, Formula};
+use super::ast::{AggFunc, BinOp, Expr, Filter, Formula};
 
 /// Parse a formula string like "Profit = Revenue - Cost"
 /// or "Tax = Revenue * 0.08 WHERE Region = \"East\""
@@ -161,13 +161,13 @@ fn parse_add_sub(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     let mut left = parse_mul_div(tokens, pos)?;
     while *pos < tokens.len() {
         let op = match &tokens[*pos] {
-            Token::Plus => "+",
-            Token::Minus => "-",
+            Token::Plus  => BinOp::Add,
+            Token::Minus => BinOp::Sub,
             _ => break,
         };
         *pos += 1;
         let right = parse_mul_div(tokens, pos)?;
-        left = Expr::BinOp(op.to_string(), Box::new(left), Box::new(right));
+        left = Expr::BinOp(op, Box::new(left), Box::new(right));
     }
     Ok(left)
 }
@@ -176,13 +176,13 @@ fn parse_mul_div(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     let mut left = parse_pow(tokens, pos)?;
     while *pos < tokens.len() {
         let op = match &tokens[*pos] {
-            Token::Star => "*",
-            Token::Slash => "/",
+            Token::Star  => BinOp::Mul,
+            Token::Slash => BinOp::Div,
             _ => break,
         };
         *pos += 1;
         let right = parse_pow(tokens, pos)?;
-        left = Expr::BinOp(op.to_string(), Box::new(left), Box::new(right));
+        left = Expr::BinOp(op, Box::new(left), Box::new(right));
     }
     Ok(left)
 }
@@ -192,7 +192,7 @@ fn parse_pow(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     if *pos < tokens.len() && tokens[*pos] == Token::Caret {
         *pos += 1;
         let exp = parse_unary(tokens, pos)?;
-        return Ok(Expr::BinOp("^".to_string(), Box::new(base), Box::new(exp)));
+        return Ok(Expr::BinOp(BinOp::Pow, Box::new(base), Box::new(exp)));
     }
     Ok(base)
 }
@@ -298,30 +298,30 @@ fn parse_comparison(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     let left = parse_add_sub(tokens, pos)?;
     if *pos >= tokens.len() { return Ok(left); }
     let op = match &tokens[*pos] {
-        Token::Eq => "=",
-        Token::Ne => "!=",
-        Token::Lt => "<",
-        Token::Gt => ">",
-        Token::Le => "<=",
-        Token::Ge => ">=",
+        Token::Eq => BinOp::Eq,
+        Token::Ne => BinOp::Ne,
+        Token::Lt => BinOp::Lt,
+        Token::Gt => BinOp::Gt,
+        Token::Le => BinOp::Le,
+        Token::Ge => BinOp::Ge,
         _ => return Ok(left),
     };
     *pos += 1;
     let right = parse_add_sub(tokens, pos)?;
-    Ok(Expr::BinOp(op.to_string(), Box::new(left), Box::new(right)))
+    Ok(Expr::BinOp(op, Box::new(left), Box::new(right)))
 }
 
 #[cfg(test)]
 mod tests {
     use super::parse_formula;
-    use crate::formula::{Expr, AggFunc};
+    use crate::formula::{AggFunc, BinOp, Expr};
 
     #[test]
     fn parse_simple_subtraction() {
         let f = parse_formula("Profit = Revenue - Cost", "Measure").unwrap();
         assert_eq!(f.target, "Profit");
         assert_eq!(f.target_category, "Measure");
-        assert!(matches!(f.expr, Expr::BinOp(ref op, _, _) if op == "-"));
+        assert!(matches!(f.expr, Expr::BinOp(BinOp::Sub, _, _)));
     }
 
     #[test]