Add hover to add_annotation

[larsoner]

Implemented something I've wanted for a while, hover support for printing label names:

import mne
brain = mne.viz.Brain('fsaverage', surf="inflated")
brain.add_annotation('HCPMMP1_combined', hover=True)

Screencast.From.2026-05-28.14-22-14.mp4

Along the way:

• Removed our custom _read_annot code, we should use nibabel for this
• Removed support for passing (labels, ctab) as annot in add_annotation. I strongly suspect it wasn't really used in practice, and wasn't tested, so I think it's safe enough.

[drammock]

do we (someday) want to provide other info on hover, besides the label name? e.g, value of the plotted STC at the hovered vertex, or MNI coords, or...

If so, we should consider whether setting up this param as a boolean is the most forward-looking choice.

[larsoner]

Hmm... probably not for add_annotation (here). Something like Brain itself, or add_data might want per-vertex hover support, which we would add with a different kwarg elsewhere I think

[drammock]

ah right of course.