feat: Pluto RX hw-gain control + AD9361 cold-init (FIR via libad9361)

- RX hw-gain (Pluto/AD936x): backend SetRxGain(ModeIdx, GainDb) — gain_control_mode
  (manual/fast_attack/slow_attack/hybrid) + hardwaregain. Controller FRxGainMode/
  FRxGainDb, commands SetRxGainMode/SetRxGain/RxGainBy, rfRxGain event, snapshot,
  applied in StartRunning. Persisted per-device (rx_gain_mode/rx_gain_db). HPSDR
  unaffected (backend no-op).
- UI: separate "RF AGC" panel below the (unchanged) WDSP AGC block, shown only for
  Pluto — FAST/SLOW/HYB/MAN mode buttons + manual GAIN slider. LayoutLeftPanel
  reflows lower panels via RelayoutBelowBands; no layout shift on HPSDR.
- Cold-init fix: direct sampling_frequency write doesn't load the AD9361 FIR
  decimation filter, so a cold-booted Pluto showed a wide centre spike that didn't
  track tuning (worked only after SDR Console pre-initialised it). Add optional
  libad9361 binding (dynamic) and call ad9361_set_bb_rate() in ApplySampleRate,
  which configures BBPLL + loads the FIR; falls back to direct write if absent.

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

Settings.pas

@@ -218,6 +218,8 @@ type
     LastBand:         Integer;
     LastXvtr:         Integer;   // -1 = HF, 0..CFG_XVTR_COUNT-1 = активный трансвертер при выходе
     SampleRate:       Integer;   // глобальный — один для всех диапазонов
+    RxGainMode:       Integer;   // Pluto: 0=manual,1=fast,2=slow,3=hybrid
+    RxGainDb:         Integer;   // Pluto: manual gain дБ
     WindowLeft:       Integer;
     WindowTop:        Integer;
     WindowWidth:      Integer;
@@ -401,6 +403,7 @@ begin
   G.WfAGCEnabled := True;
   G.WfNFEnabled := False;
   G.SampleRate := 192000;
+  G.RxGainMode := 2; G.RxGainDb := 40;
   G.PAMaxPower := 100.0;
   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;
@@ -597,6 +600,8 @@ begin
   G.LastBand         := JI(GObj,'last_band',5);
   G.LastXvtr         := JI(GObj,'last_xvtr',-1);
   G.SampleRate       := JI(GObj,'sample_rate',192000);
+  G.RxGainMode       := JI(GObj,'rx_gain_mode',2);
+  G.RxGainDb         := JI(GObj,'rx_gain_db',40);
   // Display settings
   G.FFTSize      := JI(GObj,'fft_size',131072);
   G.WindowType   := JI(GObj,'win_type',2);
@@ -712,6 +717,8 @@ begin
   JW(O,'last_band',G.LastBand);
   JW(O,'last_xvtr',G.LastXvtr);
   JW(O,'sample_rate',G.SampleRate);
+  JW(O,'rx_gain_mode',G.RxGainMode);
+  JW(O,'rx_gain_db',G.RxGainDb);
   // Display settings
   JW(O,'fft_size',G.FFTSize);
   JW(O,'win_type',G.WindowType);