panel_axes ylabel on bottom panel overwrites parent axes ylabel

[kinyatoride]

Description

Setting ylabel on a bottom panel (created via panel_axes('bottom')) overwrites the ylabel of the parent main axes, and the panel itself ends up with no ylabel. The same symmetric issue should occur for xlabel on left/right panels.

Minimal reproducer

import ultraplot as uplt

fig, axs = uplt.subplots(ncols=2, sharey=0)
pxs = axs.panel_axes('bottom')

axs.format(ylabel='ylabel')
pxs.format(ylabel='plabel')

Expected

• Both main axes show "ylabel".
• Both bottom panels show "plabel".

Actual

• At least one main axis displays "plabel" (the panel's text, transferred up).
• Both panels display no ylabel.

Environment

• ultraplot 2.1.5
• matplotlib 3.10.8
• Python 3.10.20

Likely cause

In ultraplot/axes/cartesian.py::_apply_axis_sharing_for_axis, when panel_group is true the sharing level is forced to 3, which triggers labels._transfer_label(axis.label, shared_axis_obj.label) — moving the panel's ylabel text onto the parent and hiding the panel's label. For a bottom panel, the Y-axis is not geometrically aligned with the parent's Y-axis, so this transfer should not happen.

Intended gating:

• Y-label transfer only for left/right panels
• X-label transfer only for top/bottom panels

Workaround note: pax.yaxis.label.set_visible(True) after format() does not help because the text has already been moved off the panel's Text object by _transfer_label (it calls src.set_text("")).