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/linter.rs

@@ -510,7 +510,7 @@ impl Linter {
                     self.collect_refs_expr(&arm.body);
                 }
             }
-            Expr::Field { object, .. } => {
+            Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => {
                 self.collect_refs_expr(object);
             }
             Expr::Record { fields, .. } => {