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>

RadioBackend.pas

@@ -144,6 +144,11 @@ type
     // RX-усиление (Pluto/AD936x). ModeIdx: 0=manual, 1=fast_attack, 2=slow_attack,
     // 3=hybrid. GainDb применяется только в manual. HPSDR — no-op.
     procedure SetRxGain(ModeIdx: Integer; GainDb: Integer); virtual;
+    // TX-аттенюация выходного тракта (Pluto/AD936x): hardwaregain на voltage0
+    // (output), дБ в диапазоне ~ -89.75..0 (0 = макс. мощность). Контроллер ставит
+    // максимум на приёме (≈выкл) и значение от drive на передаче. HPSDR — no-op
+    // (мощность там через DriveLevel/DUC). Db < 0.
+    procedure SetTxAtten(Db: Double); virtual;
     procedure SetAlexConfig(const A: TAlexSettings); virtual;
     procedure SetXvtrMode(AEnable, ADisablePA: Boolean; ARxAnt: Byte); virtual;
     procedure SendDDCAudio(const LeftRight: array of SmallInt); virtual;
@@ -250,4 +255,8 @@ begin
   Result := False;
 end;
 
+procedure TRadioBackend.SetTxAtten(Db: Double);
+begin
+end;
+
 end.