Create coordinate bounds on the internal-calculation path (WCRP ATTR001)

[rhaegar325]

areacella was written with no lat_bnds/lon_bnds and no bounds attribute on either coordinate, failing two weight-3 WCRP checks:

[ATTR001] Required variable 'lat' attribute 'bounds' is missing.
[ATTR001] Required variable 'lon' attribute 'bounds' is missing.

The missing attribute was a symptom: the bounds variables were never created. select_and_process_variables returns early for calculation.type == "internal", skipping the
_get_required_bounds_variables / calculate_missing_bounds_variables pair that every other path reaches. The vocabulary was already correct — asked directly it reports must_have_bounds='yes' for both axes and bnds_required = ['lon_bnds', 'lat_bnds'] — the answer just had no consumer. reorder() already reserves slots for lat_bnds/lon_bnds, so their absence was the anomaly, not the design.

Separately, calculate_areacella builds a full (145, 2) lat_bnds array to derive the cell areas and then discards it, and never computes lon_bnds at all. Rather than return those, call the existing bounds machinery before the early return: it fixes the class rather than the one variable, and calculate_longitude_bounds already unwraps the 0°-crossing first cell (#635), so no new bounds arithmetic is introduced.

areacella is currently the only internal-calculation entry in the ESM1-5 and ESM1-6 mappings. ocean.py and sea_ice.py carry the same early return, but have no internal variables to exercise it; left alone rather than changing untestable paths.

Verified end to end: lat_bnds/lon_bnds are now bit-identical to orog's on the shared N96 grid (first lon cell [-0.9375, 0.9375], not the wrapped form), both ATTR001 bounds checks go [0, 1] -> [1, 1], and the two [VAR012] bounds-consistency checks — which could not run before — pass. orog output is byte-for-byte unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.3%. Comparing base (9d78ebd) to head (4a37c51).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #647     +/-   ##
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+ Coverage   77.2%   77.3%   +0.1%     
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  Files         40      40             
  Lines       8522    8524      +2     
  Branches    1593    1593             
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+ Hits        6583    6590      +7     
+ Misses      1610    1604      -6     
- Partials     329     330      +1     

Flag	Coverage Δ
unit	77.3% <100.0%> (+0.1%)	⬆️

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