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/ast.rs

@@ -499,6 +499,12 @@ pub enum Expr {
         field: Ident,
         span: Span,
     },
+    /// Tuple index access: tuple.0, tuple.1
+    TupleIndex {
+        object: Box<Expr>,
+        index: usize,
+        span: Span,
+    },
     /// Lambda: fn(x, y) => x + y  or  fn(x: Int): Int => x + 1
     Lambda {
         params: Vec<Parameter>,
@@ -563,6 +569,7 @@ impl Expr {
             Expr::Call { span, .. } => *span,
             Expr::EffectOp { span, .. } => *span,
             Expr::Field { span, .. } => *span,
+            Expr::TupleIndex { span, .. } => *span,
             Expr::Lambda { span, .. } => *span,
             Expr::Let { span, .. } => *span,
             Expr::If { span, .. } => *span,