Keep seam faces in polygon plots

[rajeeja]

Summary

• use split seam handling by default for shaded polygon plots
• add ne30pg2 coverage for seam-crossing faces

Closes #1518

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Pull request overview

This PR addresses seam/antimeridian artifacts in face-centered shaded polygon plots by changing the default periodic-element handling to preserve seam-crossing faces, and adds regression coverage for the NE30PG2 SCRIP tutorial dataset.

Changes:

• Switch UxDataArray.plot.polygons() (and thus UxDataArray.plot() for face-centered data) to default periodic_elements="split" so seam-crossing faces are not dropped.
• Add a NE30PG2 regression test intended to ensure antimeridian faces are preserved when using split handling.
• Update plotting documentation to reflect the new default and show explicit exclude vs split examples.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
uxarray/plot/accessor.py	Changes default polygon plotting behavior to split periodic (seam-crossing) faces and updates docstring text.
test/core/test_dataarray.py	Adds a NE30PG2 regression test for split periodic handling (with a few correctness/naming adjustments needed).
docs/user-guide/plotting.ipynb	Updates user-guide narrative and examples to match the new default and recommended options.
docs/user-guide/mpl.ipynb	Clarifies Matplotlib to_polycollection() default periodic handling in the docs.

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[erogluorhan]

Thanks for doing this! It makes sense to me. For very large datasets though, this would lead to performance issues, but I think it is reasonable to expect the user to give the docs a read first to choose the performant way.