tifs_to_geozarr: propagate GDAL Unit Type + long_name into zarr attrs

[scottstanie]

Summary

When bowser tifs-to-geozarr converts a stack of GeoTIFFs whose band has a GDAL Unit Type set (e.g. dolphin's velocity.tif → m/year), that unit was getting dropped on the floor. Every variable in the output zarr ended up with just {grid_mapping: spatial_ref} attrs.

Meanwhile the backend has been surfacing attrs["units"] as dsInfo.unit on /datasets for a while, and the frontend ColormapBar has a unit slot next to the colorbar that was always empty because no variable ever set the attr.

What changes

• _Loaded dataclass gains a units: str | None field.
• _load_group reads src.units[0] from the first file of the group at the same time it sniffs dtype (one extra line, same rasterio context). Empty-string units normalised to None.
• _write_variable_to_all_levels stamps da.attrs["units"] when present and always stamps da.attrs["long_name"] = lv.display_name. Both are CF-conventions attrs, so other zarr readers benefit too.

Side effect: the colorbar title now reads Velocity (the RasterGroup's display_name) instead of the sanitized zarr variable name velocity.

Verified

Against /Volumes/.../velocity.tif (GDAL Unit Type: m/year):

>>> lv = _load_group({'name': 'Velocity', 'file_list': [velocity_tif]}, ref)
>>> lv.name, lv.display_name, lv.units
('velocity', 'Velocity', 'm/year')

Caveat

Existing cubes must be rebuilt with bowser tifs-to-geozarr to pick up the new attrs — this is a write-time fix, not a back-fill.

Test plan

• _load_group returns the expected units on a real file (see above)
• ruff / ruff-format / mypy pass
• Reporter to rebuild a cube and confirm the colorbar shows m/year under the velocity bar in the UI

🤖 Generated with Claude Code