Fix int64 ocean grid index coordinates breaking THREDDS access

[rbeucher]

Summary

• Paola reported that Laurie's published ACCESS-ESM1.5 ocean CMIP6Plus files fail on THREDDS' OPeNDAP and NCSS services.
• Of the three issues she flagged, two (the coordinates attribute pointing at native grid vars like geolon_c/geolat_c, and spurious bnds/vertices coordinate variables) were already fixed on main in earlier PRs.
• The third — index/bounds variables written as int64 — is fixed here for the remaining case: the ocean grid's i/j index coordinates, built via a bare np.arange() in ocean_supergrid.py, defaulted to the platform int (int64 on 64-bit Linux) with nothing downstream casting them back to int32.

Test plan

• Added a regression test asserting i/j are int32 in tests/unit/test_supergrid.py
• pixi run -e test python -m pytest tests/unit — 1790 passed, 2 skipped

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.2%. Comparing base (3ccc6ca) to head (549b88b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #638   +/-   ##
=====================================
  Coverage   77.2%   77.2%           
=====================================
  Files         40      40           
  Lines       8514    8514           
  Branches    1590    1590           
=====================================
  Hits        6575    6575           
  Misses      1610    1610           
  Partials     329     329           

Flag	Coverage Δ
unit	77.2% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.