Fix: decouple annotation xy from mutable inputs (swev-id: matplotlib__matplotlib-26466)

[casey-brooks]

Closes #72

Summary

• tuple-copy annotation xy in _AnnotationBase.__init__
• tuple-copy OffsetFrom reference coordinates for consistency
• add mutation regression tests and an Agg reproduction script

Reproduction

1. Run python doc/devel/examples/repro_annotate_xy_mutability.py
2. On affected builds the "After mutation" log shows annotation pixels and xytext following the mutated NumPy/list sources instead of the original coordinates (Agg backend).

Observed: annotation and text offset drift to the mutated values even though no setter was called.
Expected: annotation remains tied to its original coordinates regardless of external mutations.

Testing

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest lib/matplotlib/tests/test_text.py -q -k "annotation_xy_numpy_source_mutation_no_effect or annotation_xytext_numpy_source_mutation_no_effect or annotation_xy_list_source_mutation_no_effect or offsetfrom_ref_coord_numpy_mutation_no_effect"

Notes

• Functional bug; no stack trace was reported.
• CI should not be manually triggered.

[casey-brooks]

Local verification

• Tests: MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest lib/matplotlib/tests/test_text.py -q -k "annotation_xy_numpy_source_mutation_no_effect or annotation_xytext_numpy_source_mutation_no_effect or annotation_xy_list_source_mutation_no_effect or offsetfrom_ref_coord_numpy_mutation_no_effect" → 4 passed, 115 deselected
• Lint: .venv/bin/flake8 lib/matplotlib/text.py lib/matplotlib/tests/test_text.py doc/devel/examples/repro_annotate_xy_mutability.py → no issues

[noa-lucent]

Looks great: tuple-copying the annotation anchors fixes the mutable input regression and the new tests cover NumPy and list sources, plus OffsetFrom is now hardened. Thanks for the thorough repro script as well.