fix(hpsdr): prevent false connection lost on tick race

The RX timeout check sampled GetTickCount64 on the UI thread while the UDP receive thread could update FRXLastPktTime a millisecond later. Both values are QWord, so subtracting the newer packet tick from the older UI tick underflowed to a very large value and falsely satisfied the 3-second timeout.

Snapshot the packet timestamp once for each timeout check and require NowTick >= LastPktTick before calculating the elapsed interval. The initial-packet timeout uses the same snapshot. This preserves real timeout detection while ignoring a packet timestamp that is marginally newer than the UI tick.

Diagnostics confirmed that DDC UDP packets, callbacks, and HP status remained continuous during the reported failures; the issue was local timeout arithmetic rather than network loss or radio stream interruption.

MainForm.pas

@@ -3409,6 +3409,7 @@ procedure TMainForm.SpectrumTimerTick(Sender: TObject);
 var
   NowTick:     QWord;
   ElapsedTick: QWord;
+  LastPktTick: QWord;
   I:           Integer;
 begin
   // «×» шапки пана: закрытие отложено сюда (нельзя free панель из её кнопки).
@@ -3438,8 +3439,12 @@ begin
   begin
     NowTick     := GetTickCount64;
     ElapsedTick := NowTick - FController.FRXStartTime;
+    // Сетевой поток может обновить FRXLastPktTime на 1 ms ПОСЛЕ NowTick.
+    // QWord-вычитание в таком случае underflow'ится в огромное значение и
+    // давало ложный "Connection lost" при совершенно живом DDC-потоке.
+    LastPktTick := FController.FRXLastPktTime;
     // Ждём первый пакет 5 секунд после старта
-    if (FController.FRXLastPktTime = 0) and (ElapsedTick > 5000) then
+    if (LastPktTick = 0) and (ElapsedTick > 5000) then
     begin
       BtnStartStop.Caption := 'START';
       StyleButton(BtnStartStop, False);
@@ -3451,7 +3456,8 @@ begin
       Exit;
     end;
     // После первого пакета — следим чтобы поток не прерывался более 3 сек
-    if (FController.FRXLastPktTime > 0) and ((NowTick - FController.FRXLastPktTime) > 3000) then
+    if (LastPktTick > 0) and (NowTick >= LastPktTick) and
+       ((NowTick - LastPktTick) > 3000) then
     begin
       BtnStartStop.Caption := 'START';
       StyleButton(BtnStartStop, False);