Spot mirroring drew the same station two or three times, a few hertz apart.
Not a loop in the spot pipeline — one cluster line produces exactly one
SendSpot. It is that a popular DX station IS spotted two or three times, by
different operators inside the same minute, and no two of them agree on the
frequency to better than a few tens of hertz. This backend assumed ExpertSDR
replaced a spot bearing a callsign it already held, and said so in a comment;
it keys on the frequency too, so each of those became its own marker and stayed.
The FlexRadio backend has always done this properly (spot remove before spot
add, "one live spot per call"). TCI now does the same: the frequency drawn for
each call is remembered, a re-spot within 500 Hz sends nothing at all, and one
further away deletes the old marker before drawing the new one — so a station
that really moves still moves, instead of collecting markers.
SPOT_DELETE was not in the protocol document this backend was written from.
Confirmed before use against ars-ka0s/eesdr-tci, an independent TCI library that
lists SPOT with 5 arguments, SPOT_DELETE with 1 and SPOT_CLEAR with 0 — the
other two matching exactly what already works here, which is what makes the
third trustworthy.
The memory is cleared where Connect sends spot_clear. Left standing, it would
suppress the next spot for every remembered call as "already drawn" onto a
panorama the radio had just emptied — spots would quietly stop appearing after
a reconnect until each station changed frequency.
Five tests on the decision, which is split out from SendSpot so it can be
checked without a radio.
