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>

Settings.pas

@@ -261,6 +261,7 @@ type
     FreqMhzDigits:     Integer;  // minimum MHz digit count: 3=999MHz, 4=9.999GHz, 5=99.999GHz
     // --- PA (Power Amplifier) settings ---
     PAMaxPower:        Double;   // максимальная выходная мощность, Вт (5..200)
+    PlutoTxMaxAttDb:   Double;   // Pluto: TX-аттенюация на drive=100%, дБ<0 (дефолт -10)
     PABandCal:         array[0..CFG_BAND_COUNT-1] of Double; // калибровка на диапазон 38.8..100.0
     VHFBandCal:        array[0..CFG_XVTR_COUNT-1] of Double; // калибровка для XVTR слотов 38.8..100.0
     // --- ADC settings ---
@@ -410,6 +411,7 @@ begin
   G.SampleRate := 192000;
   G.RxGainMode := 2; G.RxGainDb := 40;
   G.PAMaxPower := 100.0;
+  G.PlutoTxMaxAttDb := -10.0;
   for i := 0 to CFG_BAND_COUNT-1 do G.PABandCal[i] := CFG_PA_BAND_CAL_DEFAULT[i];
   for i := 0 to CFG_XVTR_COUNT-1 do G.VHFBandCal[i] := CFG_VHF_CAL_DEFAULT;
   // Display defaults
@@ -665,6 +667,7 @@ begin
   G.FreqMhzDigits := EnsureRange(JI(GObj,'freq_mhz_digits',3), 3, 5);
   // PA settings
   G.PAMaxPower := JD(GObj,'pa_max_power',100.0);
+  G.PlutoTxMaxAttDb := JD(GObj,'pluto_tx_max_att',-10.0);
   for i := 0 to CFG_BAND_COUNT-1 do
     G.PABandCal[i] := JD(GObj,'pa_band_cal_'+IntToStr(i), CFG_PA_BAND_CAL_DEFAULT[i]);
   for i := 0 to CFG_XVTR_COUNT-1 do
@@ -757,6 +760,7 @@ begin
   JW(O,'freq_mhz_digits',G.FreqMhzDigits);
   // PA settings
   JW(O,'pa_max_power',G.PAMaxPower);
+  JW(O,'pluto_tx_max_att',G.PlutoTxMaxAttDb);
   for i := 0 to CFG_BAND_COUNT-1 do
     JW(O,'pa_band_cal_'+IntToStr(i),G.PABandCal[i]);
   for i := 0 to CFG_XVTR_COUNT-1 do