fix(tci): one marker per callsign on the panorama, not one per spotter

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.

changelog.json

@@ -10,7 +10,8 @@
       "Selecting a QSO shows the entity the QSO records, not one re-derived from its callsign — a 3Y0K contact logged as Bouvet showed the Antarctica matrix.",
       "Back-entering a QSO resolves the ClubLog exception at the CONTACT’S date, so a DXpedition entered months later gets the entity it had then.",
       "QRZ.com sends an island reference for an operator on one, and OpsLog read past it — it now fills the IOTA award reference before the QSO is logged.",
-      "Sync across PCs: point every OpsLog at one folder you already synchronise and your contacts follow you between machines."
+      "Sync across PCs: point every OpsLog at one folder you already synchronise and your contacts follow you between machines.",
+      "TCI: a station spotted by several operators is drawn once on the panorama instead of two or three times a few hertz apart."
     ],
     "fr": [
       "Ouvrir le panneau Awards ne tire plus plusieurs fois le journal entier en même temps — un gros log occupait brièvement des gigaoctets de mémoire.",
@@ -20,7 +21,8 @@
       "Sélectionner un QSO affiche l’entité que le QSO enregistre, pas une recalculée depuis l’indicatif — un 3Y0K logué Bouvet montrait la matrice Antarctique.",
       "Saisir un QSO a posteriori résout l’exception ClubLog à la date DU CONTACT : une DXpedition entrée des mois après retrouve l’entité qu’elle avait alors.",
       "QRZ.com envoie la référence d’île d’un opérateur sur une île, et OpsLog l’ignorait — elle remplit désormais la référence IOTA avant l’enregistrement du QSO.",
-      "Synchro entre PC : fais pointer chaque OpsLog vers un dossier déjà synchronisé et tes contacts te suivent d’une machine à l’autre."
+      "Synchro entre PC : fais pointer chaque OpsLog vers un dossier déjà synchronisé et tes contacts te suivent d’une machine à l’autre.",
+      "TCI : une station spottée par plusieurs opérateurs n’est tracée qu’une fois sur le panorama, au lieu de deux ou trois fois."
     ]
   },
   {