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/parser.rs

@@ -1646,6 +1646,20 @@ impl Parser {
             } else if self.check(TokenKind::Dot) {
                 let start = expr.span();
                 self.advance();
+
+                // Check for tuple index access: expr.0, expr.1, etc.
+                if let TokenKind::Int(n) = self.peek_kind() {
+                    let index = n as usize;
+                    self.advance();
+                    let span = start.merge(self.previous_span());
+                    expr = Expr::TupleIndex {
+                        object: Box::new(expr),
+                        index,
+                        span,
+                    };
+                    continue;
+                }
+
                 let field = self.parse_ident()?;
 
                 // Check if this is an effect operation: Effect.operation(args)
@@ -1681,11 +1695,14 @@ impl Parser {
 
     fn parse_args(&mut self) -> Result<Vec<Expr>, ParseError> {
         let mut args = Vec::new();
+        self.skip_newlines();
 
         while !self.check(TokenKind::RParen) {
             args.push(self.parse_expr()?);
+            self.skip_newlines();
             if !self.check(TokenKind::RParen) {
                 self.expect(TokenKind::Comma)?;
+                self.skip_newlines();
             }
         }