feat: Pluto TX path — attenuation-based power, full-duplex IQ stream

TX path for Pluto/AD936x (RX was already working):
- PlutoBackend: continuous full-duplex TX stream (cf-ad9361-dds-core-lpc)
  with FIFO + push thread; SendDUCIQ upsamples WDSP 192k -> device rate
  (integer, linear interp), 24->16 bit, Q inverted; zeros on underrun.
- Power via output attenuator (ADI-recommended): SetTxAtten writes
  hardwaregain on voltage0(out). Controller maps drive% -> dB
  (PlutoTxAttForDrive/ApplyPlutoTxAtten); -89.75 dB on RX (~off),
  ceiling PlutoTxMaxAttDb on 100% drive.
- Configurable max-att (default -10 dB, ADI-linear) for external amp:
  persisted in global settings, UI spin on PA page, routed through
  controller SetPlutoTxMaxAtt (UI stays decoupled from logic).
- RadioBackend: virtual SetTxAtten (no-op for HPSDR).
- Update doc/PLUTO_INTEGRATION_PLAN.md: phases 3/4/5 + risks.

Builds clean (headless + qt6 GUI). Not yet verified on hardware.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SettingsForm.pas

@@ -35,6 +35,8 @@ type
   // PA: MaxPower в Вт, BandCal[0..10] — калибровка 38.8..100.0 на диапазон
   TOnPASettingsChange = procedure(MaxPower: Double;
                                   const BandCal: array of Double) of object;
+  // Pluto TX: аттенюация выходного тракта на drive=100% (потолок), дБ<0
+  TOnPlutoTxChange = procedure(MaxAttDb: Double) of object;
   // VHF (XVTR) PA calibration: 8 slots, 38.8..100.0
   TOnVHFCalChange = procedure(const VHFCal: array of Double) of object;
   // TX-настройки целиком (любая правка → один callback с актуальной структурой)
@@ -132,6 +134,7 @@ type
 
     // ---- PA Settings tab controls ----
     FEdMaxPower:     TFlatSpinEdit;
+    FEdPlutoTxMaxAtt: TFlatFloatSpinEdit;   // Pluto TX att на drive=100%
     FEdBandCal:      array[0..10] of TFlatFloatSpinEdit;
     FEdVHFCal:       array[0..CFG_XVTR_COUNT-1] of TFlatFloatSpinEdit;
     FVHFCalLabels:   array[0..CFG_XVTR_COUNT-1] of TLabel;
@@ -265,6 +268,7 @@ type
     // ---- Callbacks ----
     FOnCATChange:        TOnCATChange;
     FOnPAChange:         TOnPASettingsChange;
+    FOnPlutoTxChange:    TOnPlutoTxChange;
     FOnVHFCalChange:     TOnVHFCalChange;
     FOnADCChange:        TOnADCChange;
     FOnWebSettingsChange: TOnWebSettingsChange;
@@ -344,6 +348,7 @@ type
     procedure OnLightThemeChkChange(Sender: TObject);
     procedure OnFreqMhzDigitsCmbChange(Sender: TObject);
     procedure OnMaxPowerChange(Sender: TObject);
+    procedure OnPlutoTxMaxAttChange(Sender: TObject);
     procedure OnBandCalChange(Sender: TObject);
     procedure OnVHFCalEdit(Sender: TObject);
     procedure OnCATSerialChange(Sender: TObject);
@@ -386,6 +391,7 @@ type
     procedure LoadLightTheme(ALight: Boolean);
     procedure LoadFreqMhzDigits(Digits: Integer);
     procedure LoadPASettings(MaxPower: Double; const BandCal: array of Double);
+    procedure LoadPlutoTxMaxAtt(MaxAttDb: Double);
     procedure LoadVHFCalSettings(const VHFCal: array of Double);
     procedure LoadCATSettings(
       const SerEnabled: array of Boolean;
@@ -404,6 +410,7 @@ type
     property OnAlexChange:       TOnAlexChange           read FOnAlexChange      write FOnAlexChange;
     property OnXvtrChange:       TOnXvtrChange           read FOnXvtrChange      write FOnXvtrChange;
     property OnPAChange:         TOnPASettingsChange     read FOnPAChange        write FOnPAChange;
+    property OnPlutoTxChange:    TOnPlutoTxChange        read FOnPlutoTxChange   write FOnPlutoTxChange;
     property OnVHFCalChange:     TOnVHFCalChange         read FOnVHFCalChange    write FOnVHFCalChange;
     property OnDisplayChange:    TOnDisplayParamChange   read FOnDisplayChange   write FOnDisplayChange;
     property OnWaterfallChange:  TOnWaterfallParamChange read FOnWaterfallChange write FOnWaterfallChange;
@@ -1662,6 +1669,29 @@ begin
   Spin.OnChange     := OnMaxPowerChange;
   FEdMaxPower       := Spin;
 
+  // Pluto TX max attenuation @ drive 100% (потолок под внешний усилитель), дБ<0
+  Lbl            := TLabel.Create(Self);
+  Lbl.Parent     := Grp;
+  Lbl.Caption    := 'Pluto TX att @100% (dB)';
+  Lbl.SetBounds(GRP_PAD + 320, R1 + 6, 180, 18);
+  Lbl.Font.Color := CLR_TEXTDIM;
+  Lbl.Font.Name  := UI_FONT;
+  Lbl.Font.Size  := 9;
+
+  FEdPlutoTxMaxAtt := TFlatFloatSpinEdit.Create(Self);
+  FEdPlutoTxMaxAtt.Parent     := Grp;
+  FEdPlutoTxMaxAtt.SetBounds(GRP_PAD + 510, R1, ED_W, BTN_H + 2);
+  FEdPlutoTxMaxAtt.Color      := CLR_INPUT;
+  FEdPlutoTxMaxAtt.Font.Color := CLR_INPUT_TEXT;
+  FEdPlutoTxMaxAtt.Font.Name  := UI_FONT;
+  FEdPlutoTxMaxAtt.Font.Size  := 9;
+  FEdPlutoTxMaxAtt.DecimalPlaces := 2;
+  FEdPlutoTxMaxAtt.MinValue   := -50;
+  FEdPlutoTxMaxAtt.MaxValue   := 0;
+  FEdPlutoTxMaxAtt.Increment  := 0.25;
+  FEdPlutoTxMaxAtt.Value      := -10;
+  FEdPlutoTxMaxAtt.OnChange   := OnPlutoTxMaxAttChange;
+
   // Calibration label
   Lbl            := TLabel.Create(Self);
   Lbl.Parent     := Grp;
@@ -2486,6 +2516,16 @@ begin
   end;
 end;
 
+procedure TSettingsForm.LoadPlutoTxMaxAtt(MaxAttDb: Double);
+begin
+  FLoading := True;
+  try
+    FEdPlutoTxMaxAtt.Value := EnsureRange(MaxAttDb, -50.0, 0.0);
+  finally
+    FLoading := False;
+  end;
+end;
+
 procedure TSettingsForm.OnMaxPowerChange(Sender: TObject);
 var
   BandCal: array[0..10] of Double;
@@ -2498,6 +2538,12 @@ begin
   FOnPAChange(FEdMaxPower.Value, BandCal);
 end;
 
+procedure TSettingsForm.OnPlutoTxMaxAttChange(Sender: TObject);
+begin
+  if FLoading then Exit;
+  if Assigned(FOnPlutoTxChange) then FOnPlutoTxChange(FEdPlutoTxMaxAtt.Value);
+end;
+
 procedure TSettingsForm.OnBandCalChange(Sender: TObject);
 var
   BandCal: array[0..10] of Double;