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