add zonal_anomaly with shared face-band weight kernel

[rajeeja]

Adds UxDataArray.zonal_anomaly() returning a same-dimension array where each face value has its latitude-band mean subtracted.

To avoid duplicating the expensive geometric intersection work already done by zonal_mean, the face-band overlap areas are extracted into a shared kernel _compute_face_band_weights(uxgrid, bands) that both zonal_mean and zonal_anomaly call. The conservative path blends band means across straddling faces using the same weight matrix; the default (non-conservative) path assigns each face to a band by centroid latitude.

Fixes #1247

[Copilot]

Pull request overview

This PR adds a new UxDataArray.zonal_anomaly() API to compute per-face zonal anomalies (subtracting each face’s latitude-band mean) and refactors conservative zonal-mean geometry into a shared face–band overlap kernel.

Changes:

• Added UxDataArray.zonal_anomaly() public method to return an anomaly field on the original unstructured grid.
• Introduced _compute_face_band_weights() to share conservative face–latitude-band overlap calculations.
• Refactored conservative zonal-mean implementation to use the shared weight kernel.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
uxarray/core/zonal.py	Adds shared face–band weight computation and implements zonal anomaly backend logic.
uxarray/core/dataarray.py	Exposes zonal_anomaly() as a new UxDataArray method.

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