gui: prune stale satellites from sky tab on each NAV-SAT update

Satellites that dropped out of a receiver's NAV-SAT report (set
below horizon, lost signal) were never removed from the sky tab's
store/trail/highlight-timer dicts, only on full receiver removal.
Over a long session this let ghost satellites accumulate without
bound, making table rebuilds and sky-plot rendering progressively
more expensive and the GUI increasingly laggy.

gui/sky_tab.py

@@ -246,8 +246,10 @@ class SkyTab(QWidget):
 
     def _on_sat_update(self, rid: str, sats: list):
         selected = self._selected_rids()
+        current_keys = set()
         for sat in sats:
             key = (rid, sat.gnss_id, sat.sv_id)
+            current_keys.add(key)
             self._store[key] = sat
             if key not in self._trail:
                 self._trail[key] = deque(maxlen=_TRAIL_MAXLEN)
@@ -255,6 +257,16 @@ class SkyTab(QWidget):
             if rid in selected:
                 self._highlight_row(key)
 
+        # Drop satellites no longer reported for this receiver so ghost
+        # entries (set below horizon, lost signal) don't accumulate forever.
+        stale = [k for k in self._store if k[0] == rid and k not in current_keys]
+        for k in stale:
+            del self._store[k]
+            self._trail.pop(k, None)
+            t = self._hi_timers.pop(k, None)
+            if t:
+                t.stop()
+
         self._sky.update_data(self._visible_sats(), self._active_trails())
         self._update_info()
         if not self._table_timer.isActive():