Feature/wfm demod: add WideFM software demodulator via DAX IQ

[ea5wa]

Summary
Adds a software FM demodulator that receives raw IQ samples from the
radio's DAX IQ RX stream, demodulates them in software, and routes the
resulting audio to a user-selected output device (e.g. a Virtual Audio
Cable such as Hi-Fi Cable Input) at 48 kHz. This makes it possible to
use the Flex radio as a wideband FM / satellite receiver feeding any
Windows audio application — in particular 9k6 packet/telemetry modems
such as hs-soundmodem.

Constitution principle honored: Principle I — the demodulator is
fully self-contained; it activates only when the operator presses the
WFM button and deactivates cleanly on toggle-off or mode change,
restoring the original IF filter without side-effects on any other
subsystem.

Audio path
Flex DAX IQ RX (channel 1, 24 kHz IQ @ 48 kHz sample rate)
└─► DaxIqWorker::processIqPacket()
└─► samplesReady(channel, QVector iqInterleaved, sampleRate)
└─► WfmDemodulator::onIqSamples()
├─► FM discriminator (atan2 phase difference)
├─► 75 µs de-emphasis filter
├─► volume scaling
└─► WaveOutWriter → selected output device @ 48 kHz Int16

On activation the IF filter is widened to ±20 kHz (G3RUH standard) and
the panadapter is centered on the slice frequency so DAX IQ RX 1 is
aligned with the signal. Both are restored on deactivation.
The audio output device is resolved via WfmDeviceDialog on first use;
the choice can be saved to AppSettings ("WfmAudioDevice") to skip the
dialog on subsequent activations.

Changes
New files:

src/core/WfmDemodulator.{h,cpp} — FM discriminator, de-emphasis,
volume control, start/stop lifecycle.
src/core/WaveInReader.{h,cpp} — Win32 waveIn capture wrapper
(reserved for loopback / future use).
src/core/WaveOutWriter.{h,cpp} — Win32 waveOut playback wrapper
used by WfmDemodulator to write PCM to the selected VAC.
src/gui/WfmDeviceDialog.{h,cpp} — output device picker dialog with
"remember my choice" checkbox.

Modified files:

src/models/DaxIqModel.{h,cpp}: expose iqSamplesReady / samplesReady
signals so the demodulator can subscribe to raw IQ without polling.
src/gui/RxApplet.{h,cpp}: WFM toggle button (36×20 px, visible
alongside the mode combo); intercepts "WFM" mode-combo selection.
src/gui/VfoWidget.{h,cpp}: WFM toggle button on the DSP/OPT tab,
visible only when the slice is in FM or NFM mode; auto-unchecks on
mode change.
src/gui/MainWindow.{h,cpp}: activateWFM / deactivateWFM methods;
wires both RxApplet and VfoWidget signals; manages filter save/restore
and panadapter centering.
CMakeLists.txt: registers the four new source files.
.gitignore: added Resumen para empezar de nuevo.txt.

Test plan
Platform: Windows 11, Flex-6600, Hi-Fi Cable (VB-Audio).
WideFM demodulation — satellite 9k6 telemetry:

Tuned Slice A to a satellite downlink frequency in FM mode.
Pressed the WFM button in the RxApplet — WfmDeviceDialog appeared;
selected Hi-Fi Cable Input and checked "remember choice".
Confirmed that:

IF filter widened to ±20 kHz automatically.
Panadapter recentered on the slice frequency.
Audio appeared on Hi-Fi Cable Input at 48 kHz.

Launched hs-soundmodem pointed at Hi-Fi Cable Output; decoded 9k6
FSK satellite telemetry frames correctly with no audio glitches.
Pressed WFM again to deactivate:

Audio stopped.
IF filter restored to its previous width.
No crashes; repeated activate/deactivate cycle 5+ times without issues.

Changed slice mode away from FM — WFM button hid automatically and
demodulator stopped cleanly.
Cleared saved device preference, reactivated — dialog appeared again
as expected.

Device error handling:

Unplugged Hi-Fi Cable, attempted activation — demodulator detected
failed open, cleared the saved preference, and returned gracefully
without crashing.