Reduce timer-tick CPU load by removing per-tick ResizeSMeter and throttling AGC line updates

- Remove ResizeSMeter (Pango font measurement + multiple SetBounds) from 60 Hz timer tick;
  it is still called on actual resize events
- Throttle UpdateAGCLines WDSP API calls from 60 Hz to ~10 Hz (every 6 ticks)
- Optimize spectrum alpha-fix loop: write-only byte access instead of read-modify-write DWORD

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

MainForm.pas

@@ -301,6 +301,7 @@ type
     // Waterfall frame timing
     FWaterfallFrameInterval: Integer;
     FWaterfallFrameCounter:  Integer;
+    FAgcLineCounter:         Integer;
     // ---- Splitter ----
     FSplitterDrag:   Boolean;
     FSplitterDragY0: Integer;
@@ -2874,7 +2875,6 @@ begin
                         ((PbWaterfall.Width <> FSpectrumWidth) or
                          (PbWaterfall.Height <> FWaterfallHeight))) then
     ResizeSpectrumPanels;
-  ResizeSMeter;
   if FSpectrumWidth <= 0 then Exit;
   SyncSpecViewFreq;
 
@@ -2920,9 +2920,14 @@ begin
     begin
       FLastSMeter := FDSPEngine.GetSMeterDBm;
       FDSPEngine.SetSpectrumWidth(FSpectrumWidth);
-      FDSPEngine.UpdateAGCLines(FSpectrumWidth);
-      FSpecView.AGCThresh    := FDSPEngine.AGCThresh;
-      FSpecView.AGCHangLevel := FDSPEngine.AGCHangLevel;
+      Inc(FAgcLineCounter);
+      if FAgcLineCounter >= 6 then
+      begin
+        FDSPEngine.UpdateAGCLines(FSpectrumWidth);
+        FSpecView.AGCThresh    := FDSPEngine.AGCThresh;
+        FSpecView.AGCHangLevel := FDSPEngine.AGCHangLevel;
+        FAgcLineCounter := 0;
+      end;
     end;
     FSpecView.WDSPReady := FWDSPReady;
     // Обновляем оверлей если видим