Fix transport and regrid for non-lat/lon grids (e.g. stereographic)

[NoraLoose]

Two related fixes to support ocean model output on projected grids (stereographic, polar, etc.) where coordinates are not standard lat/lon.

Transport (_transport.py)

T-point selection was using coordinate-value .sel() with method="nearest", which breaks on non-geographic y-coordinates like TOPAZ's stereographic y-axis (range ~−55 to 55, not −90 to 90). The fix adds a range check: if the section's lat values don't fall within the y-coordinate's range, fall back to integer .isel() instead. Also refactors the selection into a small _select() helper to avoid repeating the branch at every variable lookup.

Regrid (_regrid.py)

regrid_horizontal only handled 1-D lat/lon target grids. Extended it to accept 2-D (curvilinear) lon/lat arrays (e.g. a stereographic Arctic grid), building the xESMF target dataset with (y, x) dimensions. Conservative remapping is unsupported for curvilinear targets and falls back to bilinear with a UserWarning. Also skips the 1-D subsetting step in _subset_target for 2-D grids (ill-defined on curvilinear coordinates), and re-attaches geographic coordinates to the output after regridding since xESMF doesn't carry them through.

Tests

• New stereographic subset script and test NetCDF (tp5_stereo_subset.nc)
• test_regrid.py: round-trip regrid tests for 1-D and 2-D target grids

Checklist

• Tests added/updated for new functionality
• Documentation updated if needed
• Added an entry to docs/releases.md under [Unreleased]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.46%. Comparing base (806803d) to head (4b9ffa2).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   86.32%   86.46%   +0.14%     
==========================================
  Files          11       11              
  Lines        2120     2157      +37     
==========================================
+ Hits         1830     1865      +35     
- Misses        290      292       +2     

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