fix: preserve align label groups during pickling (swev-id: matplotlib__matplotlib-25332)

[casey-brooks]

Summary

• serialize _align_label_groups as axis indices during pickling and rebuild Grouper instances when unpickled
• drop weakref-backed groupers from the stored state and seed empty groupers when no serialized data is present
• add a regression test that exercises fig.align_labels() across a pickle round-trip

Reproduction Script

import matplotlib.pyplot as plt
import pickle

fig = plt.figure()
ax1 = fig.add_subplot(211)
ax2 = fig.add_subplot(212)

time = [0, 1, 2, 3, 4]
speed = [40000, 4300, 4500, 4700, 4800]
acc = [10, 11, 12, 13, 14]
ax1.plot(time, speed)
ax1.set_ylabel('speed')
ax2.plot(time, acc)
ax2.set_ylabel('acc')

fig.align_labels()

pickle.dumps(fig)

Observed Failure (pre-fix)

Traceback (most recent call last):
  File "<stdin>", line 19, in <module>
TypeError: cannot pickle 'weakref.ReferenceType' object

Fix Details

• map each aligned label group to the integer indices of the axes in Figure.axes
• persist the serialized mapping in Figure.__getstate__ while omitting the unpicklable groupers
• reconstruct fresh cbook.Grouper instances in Figure.__setstate__ and join axes according to the serialized indices
• ensure the serialization logic tolerates missing data for old pickles by seeding empty groupers
• add a pickle round-trip test validating the alignment metadata is retained

Files Touched

• lib/matplotlib/figure.py
• lib/matplotlib/tests/test_figure.py

References

• Resolves [Bug] Unable to pickle figure with aligned labels (Task 278) #57
• Task ID 278

[casey-brooks]

Test & Lint Summary

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest lib/matplotlib/tests/test_figure.py --maxfail=1 → 114 passed, 14 skipped
• .venv/bin/ruff check lib/matplotlib/figure.py lib/matplotlib/tests/test_figure.py → no issues

[noa-lucent]

Looks solid. getstate/setstate now serialise the alignment groups as axis indices and rebuild groupers on load, so pickle no longer trips over weakrefs. Regression test covers the round-trip and preserves group membership.