refactor(audio): replace Win32 waveOut/waveIn with QAudioSink (cross-platform)

Replace the Windows-only WaveOutWriter (waveOut API) and WaveInReader
(waveIn API) with cross-platform Qt6 equivalents so WFM demod works on
Windows, macOS, and Linux without platform guards in CMake.

Changes:
- WaveOutWriter: rewritten using QAudioSink; device selected by
  QAudioDevice::id() (persistent opaque string) instead of a
  human-readable name fragment
- WaveInReader: removed — the VITA-49 DaxIqModel::iqSamplesReady path
  is cross-platform and already works; the Win32 DAX waveIn path is no
  longer needed
- WfmDemodulator: simplified to use only the VITA-49 IQ path; updated
  all debug output to qCDebug(lcAudio)
- WfmDeviceDialog: rewritten using QMediaDevices::audioOutputs() and
  QAudioDevice::id() — works on all platforms
- WfmSettings: new header-only settings class following constitution
  Principle V (nested JSON under 'WFM' key); migrates the legacy flat
  'WfmAudioDevice' AppSettings key on first access
- CMakeLists: WFM sources moved back to unconditional CORE_SOURCES /
  GUI_SOURCES — no WIN32 guard needed
- MainWindow: resolveAudioDevice() updated to use WfmSettings API

Author: ea5wa

CMakeLists.txt

@@ -577,6 +577,8 @@ set(CORE_SOURCES
     src/core/SleepInhibitor.cpp
     src/core/MemoryCsvCompat.cpp
     src/core/MemoryRecallPolicy.cpp
+    src/core/WfmDemodulator.cpp
+    src/core/WaveOutWriter.cpp
     src/core/tnc/AetherAx25LibmodemShim.cpp
     src/core/tnc/Ax25FrameFormatter.cpp
     src/core/tnc/Ax25.cpp
@@ -634,6 +636,7 @@ set(GUI_SOURCES
     src/gui/MainWindow.cpp
     src/gui/AgcCalibrationDialog.cpp
     src/gui/AudioDeviceChangeDialog.cpp
+    src/gui/WfmDeviceDialog.cpp
     src/gui/ConnectionPanel.cpp
     src/gui/ClientDisconnectDialog.cpp
     src/gui/ConnectedStationsDialog.cpp
@@ -951,16 +954,6 @@ endif()
 
 qt_add_resources(RESOURCES resources/resources.qrc)
 
-if(WIN32)
-    list(APPEND CORE_SOURCES
-        src/core/WfmDemodulator.cpp
-        src/core/WaveOutWriter.cpp
-        src/core/WaveInReader.cpp
-    )
-    list(APPEND GUI_SOURCES
-        src/gui/WfmDeviceDialog.cpp
-    )
-endif()
 add_executable(AetherSDR ${ALL_SOURCES} ${RNNOISE_SOURCES} ${GGMORSE_SOURCES} ${RADE_SOURCES} ${BNR_SOURCES} ${SPECBLEACH_SOURCES} ${RESOURCES} ${DFNR_RESOURCES})
 
 if (USE_SYSTEM_ZLIB)