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>

RadioBackend.pas

@@ -134,6 +134,9 @@ type
     procedure SendDUCSpecific(const Pkt: TDUCSpecificPacket); virtual;
     procedure SendHighPriority(const Pkt: THighPriorityPacket); virtual;
     procedure SetStepAtten(dB: Byte); virtual;
+    // 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;
     procedure SetAlexConfig(const A: TAlexSettings); virtual;
     procedure SetXvtrMode(AEnable, ADisablePA: Boolean; ARxAnt: Byte); virtual;
     procedure SendDDCAudio(const LeftRight: array of SmallInt); virtual;
@@ -204,6 +207,10 @@ procedure TRadioBackend.SetStepAtten(dB: Byte);
 begin
 end;
 
+procedure TRadioBackend.SetRxGain(ModeIdx: Integer; GainDb: Integer);
+begin
+end;
+
 procedure TRadioBackend.SetAlexConfig(const A: TAlexSettings);
 begin
 end;