What every local client of the SCV daemon needs, without depending on the
server: the instance [Layout] (every path under SCV_HOME, and the
instance's service unit name), the delegation-depth variable a delegated SCV
inherits, framed reading and writing ([Connection], [read_frame]),
private instance files ([fs::replace_private]), [Secret] values
that never print, byte-bounded text
([text::utf8_prefix]), and [control] for daemon management requests,
which fail with a typed [ControlError].