Adds a "Horizon lock" toggle that flattens the head pose to yaw-only,
ignoring pitch (vertical) and roll (tilt). The display then only pans
horizontally as you turn your head, staying level and at a fixed height.
This is a comfort/nausea aid — particularly on glasses such as the Viture
Pro, where pulse and micro-movement jitter on the pitch/roll axes makes the
display visibly wobble (see #80, #114). Unlike smooth follow, which chases
those noisy axes, horizon lock removes them from the rendered orientation
entirely.
Implementation: a swing-twist decomposition about the NWU up-axis (Z),
applied to the pose quaternions in DeviceDataStream, so the shader,
look-ahead and follow logic all inherit the yaw-only orientation. Exposed
as a GtkSwitch in the GNOME settings (Features group) and wired through the
shared gschema.
20bd0b6435