Read-only mode guards, applied after register_all when the VM is
created with readonly = true. Mutating builtins stay registered
but are replaced with stubs that raise
readonly: <name> blocked (write operations are disabled in read-only mode),
so semi-trusted scripts fail with a clear error instead of a
nil-index. Read paths (http.get, fs.read, env.get, db.query,
systemd.list_units, …) are untouched.