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/mod.rs

@@ -1,5 +1,5 @@
 pub mod parser;
 pub mod ast;
 
-pub use ast::{AggFunc, Expr, Formula};
+pub use ast::{AggFunc, BinOp, Expr, Formula};
 pub use parser::parse_formula;