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>

MainForm.pas

@@ -498,6 +498,7 @@ type
     function  DefaultMicSource: TTXMicSource;
     procedure ApplyMOX(Active: Boolean);
     procedure OnPASettingsChange(MaxPower: Double; const BandCal: array of Double);
+    procedure OnPlutoTxMaxAttChange(MaxAttDb: Double);
     procedure OnVHFCalSettingsChange(const VHFCal: array of Double);
     procedure OnTXSettingsChange(const T: TTXSettings);
     procedure BtnAGCModeClick(Sender: TObject);
@@ -5053,6 +5054,12 @@ begin
     FController.FNetwork.UpdateState(XvtrTranslate(FController.FCenterFreq), XvtrTranslate(ActiveTXFreqHz), FController.FDriveLevel, FController.FTransmitting, True, True);
 end;
 
+procedure TMainForm.OnPlutoTxMaxAttChange(MaxAttDb: Double);
+begin
+  // Логика (clamp/apply/persist) — в контроллере; UI лишь передаёт значение.
+  FController.SetPlutoTxMaxAtt(MaxAttDb);
+end;
+
 procedure TMainForm.OnVHFCalSettingsChange(const VHFCal: array of Double);
 var i: Integer;
 begin
@@ -5708,6 +5715,7 @@ begin
     SF.OnVisibilityChange := ApplyVisibility;
     SF.OnFPSChange        := ApplyFPS;
     SF.OnPAChange         := OnPASettingsChange;
+    SF.OnPlutoTxChange    := OnPlutoTxMaxAttChange;
     SF.OnVHFCalChange     := OnVHFCalSettingsChange;
     SF.OnCATChange        := OnCATSettingsChange;
     SF.OnThemeChange           := SetLightTheme;
@@ -5726,6 +5734,7 @@ begin
     IfThen(FController.FNetwork.Connected, FController.FNetwork.Device.BoardType, FPendingBoardType));
   SF.LoadXvtrSettings(FController.FXvtrSettings);
   SF.LoadPASettings(FController.FPAMaxPower, FController.FPABandCal);
+  SF.LoadPlutoTxMaxAtt(FController.FPlutoTxMaxAttDb);
   SF.LoadVHFCalSettings(FController.FVHFBandCal);
   SF.LoadCATSettings(
     FCATLastGlobal.CATSerialEnabled,