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/lsp.rs

@@ -1576,7 +1576,7 @@ fn collect_call_site_hints(
                 collect_call_site_hints(source, e, param_names, hints);
             }
         }
-        Expr::Field { object, .. } => {
+        Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => {
             collect_call_site_hints(source, object, param_names, hints);
         }
         Expr::Run { expr, handlers, .. } => {