saveCursorOrResetMargins

Resolves parameterless CSI s against the current horizontal-margin mode (DECLRMM).

With DECLRMM disabled, saves the same state as saveCursor and returns true. Otherwise resets left/right margins to the full width, with normal DECSLRM cursor homing, and returns false. The parser saves its charset state only on true. Mode ownership stays with the sink; implementations must not cache a second mode flag.