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>
Removes dead use statements across dispatch, formula, import, model, and
UI modules. Prefixes intentionally unused variables with _ in app.rs,
analyzer.rs, and grid.rs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
