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/c_backend.rs

@@ -3237,6 +3237,11 @@ impl CBackend {
                 Ok(format!("{}.{}", obj, field.name))
             }
 
+            Expr::TupleIndex { object, index, .. } => {
+                let obj = self.emit_expr(object)?;
+                Ok(format!("{}.__{}", obj, index))
+            }
+
             Expr::Match { scrutinee, arms, .. } => {
                 self.emit_match(scrutinee, arms)
             }
@@ -4518,7 +4523,7 @@ impl CBackend {
                     || self.expr_uses_rc_vars_from_scope(right)
             }
             Expr::UnaryOp { operand, .. } => self.expr_uses_rc_vars_from_scope(operand),
-            Expr::Field { object, .. } => self.expr_uses_rc_vars_from_scope(object),
+            Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => self.expr_uses_rc_vars_from_scope(object),
             Expr::EffectOp { args, .. } => {
                 args.iter().any(|a| self.expr_uses_rc_vars_from_scope(a))
             }
@@ -4707,7 +4712,7 @@ impl CBackend {
                     self.collect_free_vars(val, bound, free);
                 }
             }
-            Expr::Field { object, .. } => {
+            Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => {
                 self.collect_free_vars(object, bound, free);
             }
             Expr::Match { scrutinee, arms, .. } => {

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, ..
             } => {