fix: ast.field not node.field in alias resolver, 37 tests passing

ReferenceError: node is not defined — was using wrong variable name.
Verified with 37 tests covering: firmware type names, aliases, route,
numeric ops, string ops, payload dot notation, hops, size, observations,
AND/OR/NOT, parentheses, and error handling.

public/packet-filter.js

@@ -251,7 +251,7 @@
           var eq;
           // Resolve type aliases (e.g., "Channel Msg" → "GRP_TXT")
           var resolvedTarget = target;
-          if (node.field === 'type' && typeof target === 'string') {
+          if (ast.field === 'type' && typeof target === 'string') {
             var alias = TYPE_ALIASES[String(target).toLowerCase()];
             if (alias) resolvedTarget = alias;
           }