Sudden-accent dynamics (fp, sf, sfz, fz, rfz, sffz) do not redefine the prevailing sustained level

fp/sf/sfz/fz/rfz/sffz are momentary accents on a single note; the underlying sustained dynamic before the accent is what continues afterward (an `fp` conventionally implies the level drops to piano immediately after the loud attack, not that the piece is now "at fp level").

Gould, Behind Bars, ch. Dynamics distinguishes sustained-level marks (ppp…fff) from sudden/local accent marks; Read, Music Notation, ch. Dynamics, gives the same split and specifically explains `fp` as forte attack immediately followed by piano.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.