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/codegen/js_backend.rs

@@ -1268,6 +1268,11 @@ impl JsBackend {
                 Ok(format!("{}.{}", obj, field.name))
             }
 
+            Expr::TupleIndex { object, index, .. } => {
+                let obj = self.emit_expr(object)?;
+                Ok(format!("{}[{}]", obj, index))
+            }
+
             Expr::Run {
                 expr, handlers, ..
             } => {