toJson(ASTNode*) recursively serializes the AST to JSON with id, concept,
properties, and children structure. Added childRoles() to ASTNode for
enumeration. FetchContent pulls nlohmann/json v3.11.3. Test serializes the
full Calculator model and verifies every node in the JSON output.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
