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/interpreter.rs

@@ -1415,6 +1415,34 @@ impl Interpreter {
                 }
             }
 
+            Expr::TupleIndex {
+                object,
+                index,
+                span,
+            } => {
+                let obj_val = self.eval_expr(object, env)?;
+                match obj_val {
+                    Value::Tuple(elements) => {
+                        if *index < elements.len() {
+                            Ok(EvalResult::Value(elements[*index].clone()))
+                        } else {
+                            Err(RuntimeError {
+                                message: format!(
+                                    "Tuple index {} out of bounds for tuple with {} elements",
+                                    index,
+                                    elements.len()
+                                ),
+                                span: Some(*span),
+                            })
+                        }
+                    }
+                    _ => Err(RuntimeError {
+                        message: format!("Cannot use tuple index on {}", obj_val.type_name()),
+                        span: Some(*span),
+                    }),
+                }
+            }
+
             Expr::Lambda { params, body, .. } => {
                 let closure = Closure {
                     params: params.iter().map(|p| p.name.name.clone()).collect(),