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>

IIOBindings.pas

@@ -84,6 +84,13 @@ var
   // ---- Утилиты ----
   iio_strerror: procedure(err: cint; dst: PAnsiChar; len: csize_t); cdecl;
 
+  // ---- libad9361 (опционально) ----
+  // ad9361_set_bb_rate: настраивает BBPLL + ЗАГРУЖАЕТ FIR-таблицу под частоту.
+  // Без него прямая запись sampling_frequency на холодную даёт мусор (битый FIR).
+  // nil если libad9361 не загружена.
+  ad9361_set_bb_rate: function(dev: Piio_device; rate: culong): cint; cdecl;
+  ad9361_set_trx_fir_enable: function(dev: Piio_device; enable: cint): cint; cdecl;
+
 // Загружает libiio (один раз). True — готово к использованию. Идемпотентно.
 function IIOLoad: Boolean;
 // True, если libiio загружена и символы разрешены.
@@ -95,16 +102,20 @@ implementation
 
 var
   GLib:    TLibHandle = NilHandle;
+  GLibAD:  TLibHandle = NilHandle;   // libad9361 (опционально)
   GLoaded: Boolean = False;
 
 const
 {$IFDEF WINDOWS}
   IIO_LIBS: array[0..1] of string = ('libiio.dll', 'iio.dll');
+  AD_LIBS:  array[0..1] of string = ('libad9361.dll', 'ad9361.dll');
 {$ELSE}
   {$IFDEF DARWIN}
   IIO_LIBS: array[0..1] of string = ('libiio.dylib', 'libiio.1.dylib');
+  AD_LIBS:  array[0..1] of string = ('libad9361.dylib', 'libad9361.0.dylib');
   {$ELSE}
   IIO_LIBS: array[0..1] of string = ('libiio.so', 'libiio.so.0');
+  AD_LIBS:  array[0..1] of string = ('libad9361.so', 'libad9361.so.0');
   {$ENDIF}
 {$ENDIF}
 
@@ -163,6 +174,21 @@ begin
     iio_buffer_step                  := Sym('iio_buffer_step');
     iio_buffer_end                   := Sym('iio_buffer_end');
     iio_strerror                     := Sym('iio_strerror');
+
+    // libad9361 — опционально (ad9361_set_bb_rate грузит FIR). Отсутствие не
+    // фатально: ApplySampleRate откатится на прямую запись sampling_frequency.
+    if GLibAD = NilHandle then
+      for I := 0 to High(AD_LIBS) do
+      begin
+        GLibAD := LoadLibrary(AD_LIBS[I]);
+        if GLibAD <> NilHandle then Break;
+      end;
+    if GLibAD <> NilHandle then
+    begin
+      ad9361_set_bb_rate        := GetProcedureAddress(GLibAD, 'ad9361_set_bb_rate');
+      ad9361_set_trx_fir_enable := GetProcedureAddress(GLibAD, 'ad9361_set_trx_fir_enable');
+    end;
+
     GLoaded := True;
   except
     on E: Exception do
@@ -190,6 +216,11 @@ begin
 end;
 
 finalization
+  if GLibAD <> NilHandle then
+  begin
+    UnloadLibrary(GLibAD);
+    GLibAD := NilHandle;
+  end;
   if GLib <> NilHandle then
   begin
     UnloadLibrary(GLib);