reproject() and merge() drop non-spatial coords; y-axis convention undocumented

[brendancol]

M-6: non-spatial coords dropped by reproject() and merge()

In xrspatial/reproject/__init__.py, the output DataArray is built with only {ydim: y_coords, xdim: x_coords} (plus the band coord on the 3D path). Any other coord on the input is dropped:

• scalar coords (e.g. a single time timestamp)
• non-dimension coords (e.g. spatial_ref from rioxarray)
• any extra dim coord that does not apply to the rebuilt grid

This affects both reproject() (around the result_data.ndim == 3 / 2D branches) and merge() (the final xr.DataArray construction).

Fix: after assigning y/x (and band) coords, copy any remaining coord whose dims do not include the spatial ydim or xdim. Coords aligned to ydim/xdim should still be dropped because their values are stale after the grid is rebuilt.

M-8: y-axis output direction is undocumented

_make_output_coords in xrspatial/reproject/_grid.py always emits y from top - res/2 down to bottom + res/2, so a y-ascending input produces a y-descending output. This is the standard raster convention (top-down, north-up) and is the right behaviour, but it is not stated in the reproject() or merge() docstrings, and the existing test_south_up_matches_north_up only checks .values equality.

Fix: add a sentence to the Returns section of both docstrings.

Plan

• Update the 2D and 3D branches of reproject() to carry forward non-spatial coords.
• Update merge() to do the same from the first input raster.
• Document the y-descending output convention in both docstrings.
• Add TestCoordsPreservation covering scalar time, non-dim string coord, stale y-aligned coord drop, band coord round-trip, merge scalar coord, and y-descending output for numpy and dask backends.