Marks content to render inside this row's marker dot - a glyph, an icon,
an avatar.
The per-row counterpart to the timeline-wide *cngxTimelineMarkerTpl, and
the only route when the row stands on its own. It also wins over that
template when both are present: most-local-wins, the same direction as
every other slot in the family, so one row can carry a different glyph
without the timeline's default being torn out for the rest.
Whatever renders here is inside an aria-hidden element - decoration only.