Accent and marcato together on one note is non-standard

Accent (`>`) and marcato (`^`) are both single "strong attack" marks of increasing intensity; no authority documents stacking both on the same notehead as a sanctioned combination the way staccato+tenuto=portato is sanctioned.

`articulations: ['accent', 'marcato']` on a single `NoteObject` (Model B/C, where `articulations` is an array) — the renderer will happily stack both an accent wedge and a marcato roof on the same notehead.

Gould, Behind Bars, ch. Articulation — enumerates the individual marks and the one recognized combination (portato); accent+marcato does not appear as a combination anywhere in Gould, Read, or Ross.

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