Rhythm Step Grid

One cycle of a rhythm as a row of steps (a note's first step in the value fill, held steps tinted, rests with an edge at least 3:1 against the card), each note one bar, a stronger edge on every beat. Click a step to add or remove a note, drag a note to set its end (committed on release, a lost pointer capture or a release outside), or Shift+arrow on it; a roving Tab stop moves with the arrows, Home and End. One commit per edit; a refused edit is announced in a status line. The row maths sit in a pure core beside it.