The marquee now has the dial it needs. `ScoreboardPresenter.PixelsPerSecond` is the single owner of
the range (20-400, default 120), the layer's properties expose it with a live px/s readout, and the
value is persisted as Scoreboard.ScrollSpeed because it is a READABILITY choice — re-dialling it
every launch would be a tax on the creator who needs the strip to be readable.
Advance() still takes an optional override so tests can drive exact distances.
Guard: the presenter test proves speed is real (same elapsed time, twice the speed, different
window) — not a value that is stored and ignored — and the panel test pins the control, its range
and the persistence key.
gramps
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