Feature request: OpenTrack integration

[Xaklse]

Description:
Add integration with OpenTrack (https://github.com/opentrack/opentrack) to allow head-tracking data to be used as input in PadForge.

Motivation:
OpenTrack is a widely used open-source head-tracking application (supports webcam, IR, and other trackers) commonly used for sim racing and flight sim titles. Native integration would let users route head-tracking data through PadForge's mapping system alongside existing gamepad/gyro inputs.

My use case:
Map head yaw rotation (with a configurable deadzone/threshold angle) to the right stick's X axis. While racing, this would let me look left/right using head movement while keeping the right thumb on the face buttons (ABXY).

Implementation notes:

• OpenTrack supports UDP output, and FreeTrack 2.0 protocol, that could serve as an input source for PadForge.
• Be aware there are other commercial programs that also emit head-tracking data following OpenTrack output options.

[hifihedgehog]

@Xaklse, this shipped on v4-dev as #355 and lands in v4.4.0.

What you get. Settings gains a Head Tracking card, off by default. Turn it on and the Devices page shows a Head Tracker row with six axes: Head Yaw, Head Pitch, Head Roll, Head X, Head Y, Head Z. Pick Head Yaw as the source for the right stick's X axis and set the deadzone on that mapping. The card's Rotation Range (degrees) sets how far you turn for full deflection.

Both of the outputs you named work, at the same time. OpenTrack's UDP over network output (set its address to 127.0.0.1, port 4242) and freetrack 2.0 Enhanced through the FT_SharedMem block, so a game that already reads FreeTrack keeps working beside PadForge. The UDP format is accepted from any sender, so the commercial trackers that emit it can point straight at PadForge on the same port.

Why this beats routing through OpenTrack's own outputs. OpenTrack on Windows has no gamepad output at all. Here the pose becomes a real controller axis with PadForge's deadzone, curve, and inversion on it, and the axes recenter one second after the tracker stops so a stick is never left pinned.

Docs: https://padforge.org/docs/features/head-tracking/. The two things deliberately left out are TrackIR's NPClient interface (a DLL games load by game ID, not a stream) and sending PadForge motion back to OpenTrack.

One thing to know: nothing ran against a live OpenTrack yet. The wire formats were built from OpenTrack's source and tested from bytes. If a roll or translation axis reads backwards for you, the per-mapping invert covers it, and a note here gets the default flipped.