The one-letter marker for a set tracked on a single side ("L" / "R"), or
null for a bilateral set — which carries no marker, since tracking both
sides at once is the default.
Takes t rather than calling useTranslation so it can be used
inside a map or a plain loop. Pair it with markSecondSides
(@enode/core/training/exercise-overview) to number a unilateral L/R pair as
one set and tell its two rows apart.
The one-letter marker for a set tracked on a single side ("L" / "R"), or
nullfor a bilateral set — which carries no marker, since tracking both sides at once is the default.