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/ast.rs

@@ -9,6 +9,25 @@ pub enum AggFunc {
     Count,
 }
 
+/// Arithmetic and comparison operators used in binary expressions.
+/// Having an enum (rather than a raw String) means the parser must
+/// produce a valid operator; invalid operators are caught at parse
+/// time rather than silently returning Empty at eval time.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum BinOp {
+    Add,
+    Sub,
+    Mul,
+    Div,
+    Pow,
+    Eq,
+    Ne,
+    Lt,
+    Gt,
+    Le,
+    Ge,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Filter {
     pub category: String,
@@ -19,7 +38,7 @@ pub struct Filter {
 pub enum Expr {
     Number(f64),
     Ref(String),
-    BinOp(String, Box<Expr>, Box<Expr>),
+    BinOp(BinOp, Box<Expr>, Box<Expr>),
     UnaryMinus(Box<Expr>),
     Agg(AggFunc, Box<Expr>, Option<Filter>),
     If(Box<Expr>, Box<Expr>, Box<Expr>),