Align spectrum grid and ruler to FM tuning step

When FM mode is active, vertical grid lines and frequency labels snap to
FM channel boundaries (FM_STEP_HZ) instead of the fixed 8-division layout.
Grid and label density auto-adapt: lines thinner than 4px are thinned,
labels are spaced so they never overlap (short tick shown between labeled
lines). Grid cache invalidates on center-freq, step, or span changes.

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

MainForm.pas

@@ -2797,6 +2797,10 @@ begin
   FSpecView.SpanHz    := FSpanHz;
   FSpecView.Mode      := FMode;
   FSpecView.FilterBW  := FFilterBW;
+  if FMode = MODE_FM then
+    FSpecView.FMGridStepHz := FM_STEP_HZ[FFMStepIdx]
+  else
+    FSpecView.FMGridStepHz := 0;
   // TX overlay следует за активной TX-частотой (учитывает split). При CTUN
   // спектр TX будет отрисован на VFO, а не в центре дисплея.
   FSpecView.TXFreq    := ActiveTXFreqHz;
@@ -4805,6 +4809,8 @@ begin
   if FBtnFMSteps[Idx] <> nil then
     FBtnFMSteps[Idx].Active := True;
   if FWebServer <> nil then FWebServer.FMStepIdx := Idx;
+  if (FMode = MODE_FM) and (FSpecView <> nil) then
+    FSpecView.FMGridStepHz := FM_STEP_HZ[Idx];
 end;
 
 procedure TMainForm.BtnFMStepClick(Sender: TObject);

SpectrumView.pas

@@ -38,6 +38,9 @@ type
     FGridBitmap:      TBitmap;
     FGridBitmapW:     Integer;
     FGridBitmapH:     Integer;
+    FFMGridLastCenter: Double;
+    FFMGridLastStepHz: Double;
+    FFMGridLastSpan:   Double;
     FRulerBitmap:     TBitmap;
     FSpecGradImg:     TLazIntfImage;
     FSpecGradBmp:     TBitmap;
@@ -79,6 +82,7 @@ type
     FSpecRefLevel:  Double;
     FSpecRange:     Double;
     FSpecGridStep:  Double;
+    FFMGridStepHz:  Double;
 
     // ── TX overlay (Thetis-style: при TX данные в FSpectrumBuf приходят
     //    от TX-анализатора @ FTXSpanHz Гц вокруг FTXFreq, и должны быть
@@ -128,6 +132,7 @@ type
     // ── Приватные методы рендеринга ───────────────────────────────────────────
     function  ActiveVfoFreq: Double;
     function  ScaleX(X, Total, Width: Integer): Integer;
+    procedure SetFMGridStepHz(V: Double);
     procedure DrawSpectrumGradient(const SpPts: array of TPoint; W, H: Integer);
     procedure DrawMarkerLine(C: TCanvas; W, H: Integer);
     // Альфа-blending вертикальной полосы поверх FSpectrumBitmap (pf32bit, BGRA).
@@ -167,6 +172,7 @@ type
     property SpecRefLevel:  Double read FSpecRefLevel  write FSpecRefLevel;
     property SpecRange:     Double read FSpecRange     write FSpecRange;
     property SpecGridStep:  Double read FSpecGridStep  write FSpecGridStep;
+    property FMGridStepHz:  Double read FFMGridStepHz  write SetFMGridStepHz;
 
     // ── TX overlay ────────────────────────────────────────────────────────────
     property TXMode:     Boolean read FTXMode     write FTXMode;
@@ -404,6 +410,8 @@ begin
   FWfIntfImg       := nil;
   FWfBitmapW       := 0; FWfBitmapH  := 0;
   FGridBitmapW     := 0; FGridBitmapH := 0;
+  FFMGridLastCenter := -1.0; FFMGridLastStepHz := -1.0; FFMGridLastSpan := -1.0;
+  FFMGridStepHz    := 0.0;
   FSpPtsLen        := 0;
   FRulerLastFreq   := -1.0; FRulerLastVfo := -1.0;
   // Водопад AGC defaults
@@ -467,6 +475,16 @@ begin
   else Result := Round(X / Total * Width);
 end;
 
+procedure TSpectrumView.SetFMGridStepHz(V: Double);
+begin
+  if Abs(FFMGridStepHz - V) > 0.5 then
+  begin
+    FFMGridStepHz := V;
+    InvalidateGridCache;
+    InvalidateRulerCache;
+  end;
+end;
+
 procedure TSpectrumView.DrawMarkerLine(C: TCanvas; W, H: Integer);
 var MX: Integer; MarkerFreq: Double; MarkerLbl: string;
 begin
@@ -734,6 +752,8 @@ var
   S0, S1: Integer;
   InvRange: Double;
   LabelBandW, SrcCount: Integer;
+  GridFreqS, GridLine, pixPerStep: Double;
+  N, gridMult: Integer;
 begin
   if FSpectrumBitmap = nil then Exit;
   W := FSpectrumBitmap.Width; H := FSpectrumBitmap.Height;
@@ -744,9 +764,16 @@ begin
   InvRange := 1.0 / (DBmax - DBmin);
 
   // ── 1. Фон+сетка из кэша ──────────────────────────────────────────────────
-  if (FGridBitmapW <> W) or (FGridBitmapH <> H) then
+  if (FGridBitmapW <> W) or (FGridBitmapH <> H) or
+     ((FFMGridStepHz > 0) and
+      ((Abs(FCenterFreq - FFMGridLastCenter) > 0.5) or
+       (FFMGridStepHz <> FFMGridLastStepHz) or
+       (Abs(FSpanHz - FFMGridLastSpan) > 1.0))) then
   begin
     FGridBitmapW := W; FGridBitmapH := H;
+    FFMGridLastCenter := FCenterFreq;
+    FFMGridLastStepHz := FFMGridStepHz;
+    FFMGridLastSpan   := FSpanHz;
     FGridBitmap.SetSize(W, H);
     GC := FGridBitmap.Canvas;
     PaintVerticalGradient(GC, W, H, FTheme.SpecGradTop, FTheme.SpecGradBot);
@@ -767,6 +794,35 @@ begin
       GC.TextOut(1, Yp - 9, Format('%4.0f', [dB]));
       dB := dB - FSpecGridStep;
     end;
+    if FFMGridStepHz > 0 then
+    begin
+      GridFreqS  := FCenterFreq - FSpanHz / 2;
+      pixPerStep := W * FFMGridStepHz / FSpanHz;
+      if pixPerStep >= 1.0 then
+        gridMult := Max(1, Ceil(4.0 / pixPerStep))
+      else
+        gridMult := 0;
+      if gridMult > 0 then
+      begin
+        N := Ceil(GridFreqS / FFMGridStepHz);
+        GridLine := N * FFMGridStepHz;
+        while GridLine <= FCenterFreq + FSpanHz / 2 + 0.5 do
+        begin
+          if (N mod gridMult) = 0 then
+          begin
+            GX := Round((GridLine - GridFreqS) / FSpanHz * W);
+            if (GX >= 0) and (GX < W) then
+            begin
+              GC.Pen.Color := FTheme.SpecGrid;
+              GC.MoveTo(GX, 0); GC.LineTo(GX, H);
+            end;
+          end;
+          Inc(N);
+          GridLine := GridLine + FFMGridStepHz;
+        end;
+      end;
+    end else
+    begin
       for i := 0 to 8 do
       begin
         GX := ScaleX(i, 8, W);
@@ -774,6 +830,7 @@ begin
         GC.MoveTo(GX, 0); GC.LineTo(GX, H);
       end;
     end;
+  end;
   LCLIntf.BitBlt(C.Handle, 0, 0, W, H, FGridBitmap.Canvas.Handle, 0, 0, $CC0020);
 
   // ── 2. Полоса фильтра ─────────────────────────────────────────────────────
@@ -1074,8 +1131,9 @@ end;
 procedure TSpectrumView.DrawRuler;
 var
   C: TCanvas; W, H, i, X: Integer;
-  FreqStart, FreqHz: Double; VfoX: Integer;
+  FreqStart, FreqHz, GridLine, pixPerStep: Double; VfoX: Integer;
   Lbl: string; TW: Integer;
+  N, labelMult: Integer;
 begin
   if FPbRuler = nil then Exit;
   W := FPbRuler.Width; H := FPbRuler.Height;
@@ -1094,6 +1152,38 @@ begin
   C.Font.Name := 'Courier New'; C.Font.Size := 7;
   C.Brush.Style := bsClear;
   FreqStart := FCenterFreq - FSpanHz / 2;
+  if FFMGridStepHz > 0 then
+  begin
+    pixPerStep := W * FFMGridStepHz / FSpanHz;
+    if pixPerStep >= 1.0 then
+      labelMult := Max(1, Ceil((C.TextWidth('000.000') + 6) / pixPerStep))
+    else
+      labelMult := MaxInt;
+    N := Ceil(FreqStart / FFMGridStepHz);
+    GridLine := N * FFMGridStepHz;
+    while GridLine <= FCenterFreq + FSpanHz / 2 + 0.5 do
+    begin
+      X := Round((GridLine - FreqStart) / FSpanHz * W);
+      if (X >= 0) and (X < W) then
+      begin
+        C.Pen.Color := FTheme.RulerBorder;
+        if (N mod labelMult) = 0 then
+        begin
+          C.MoveTo(X, 0); C.LineTo(X, H div 2);
+          Lbl := Format('%.3f', [GridLine / 1e6]);
+          TW  := C.TextWidth(Lbl);
+          C.Font.Color := FTheme.RulerText;
+          C.TextOut(X - TW div 2, H div 2 - 1, Lbl);
+        end else
+        begin
+          C.MoveTo(X, 0); C.LineTo(X, H div 4);
+        end;
+      end;
+      Inc(N);
+      GridLine := GridLine + FFMGridStepHz;
+    end;
+  end else
+  begin
     for i := 0 to 8 do
     begin
       X := ScaleX(i, 8, W);
@@ -1105,6 +1195,7 @@ begin
       C.Font.Color := FTheme.RulerText;
       C.TextOut(X - TW div 2, H div 2 - 1, Lbl);
     end;
+  end;
   VfoX := Round((ActiveVfoFreq - FCenterFreq + FSpanHz/2) / FSpanHz * W);
   if (VfoX >= 0) and (VfoX < W) then
   begin