feat: add tuple index access, multiline args, and effect unification fix

- Tuple index: `pair.0`, `pair.1` syntax across parser, typechecker,
  interpreter, C/JS backends, formatter, linter, and symbol table
- Multi-line function args: allow newlines inside argument lists
- Fix effect unification for callback parameters (empty expected
  effects means "no constraint", not "must be pure")

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

src/formatter.rs

@@ -598,6 +598,9 @@ impl Formatter {
             Expr::Field { object, field, .. } => {
                 format!("{}.{}", self.format_expr(object), field.name)
             }
+            Expr::TupleIndex { object, index, .. } => {
+                format!("{}.{}", self.format_expr(object), index)
+            }
             Expr::If { condition, then_branch, else_branch, .. } => {
                 format!(
                     "if {} then {} else {}",