HeadingUpdate should include device axis.

The HeadingUpdate should include the device axis to which it is relative to, which would allow for better local rotations which don't affect the bearing. Right now, the bearing is assumed to be relative to the device up direction <0, 1, 0>, but a rotation around the Z axis causes a change in bearing. It would be better to compute the internal bearing relative to the Z axis to avoid this.

No files or tests are named. Start by locating HeadingUpdate and the bearing calculation, then trace how the device axis is represented and how the current up-direction assumption affects rotations. Done when HeadingUpdate carries the relevant device axis and rotations around Z no longer alter the internal bearing as described.

Written by the indexing model from the issue text.