[rhaegar325]

Fix #742

Problem

With cmip_version: CMIP7, --resume never resumes. find_resume_checkpoint
derives the expected variable_id with a CMIP6-style parse:

expected_variable_id = variable.split(".", 1)[-1].split("_", 1)[0]

For ocean.wmo.tavg-ol-hxy-sea.mon.glb this gives
wmo.tavg-ol-hxy-sea.mon.glb, which never equals the file's
variable_id = 'wmo'. Every existing split is rejected, the function returns
None, and the worker reprocesses from the first input year into a new
version directory, leaving the previous partial version behind.

Observed in production: an esm-piControl wmo run with 107 complete,
marker-carrying splits in v20261006/ returned no checkpoint.

Fix

A new helper, _resume_file_identity(), returns the frequency and the global
attributes that identify a variable's output files.

• CMIP7 (realm.variable.branding.frequency.region): match on
  variable_id, branding_suffix, frequency and region.
• CMIP6: unchanged (variable_id from the table-qualified name).

Matching on variable_id alone would not be enough for CMIP7.
atmos.tas.tavg-h2m-hxy-u, atmos.tas.tmaxavg-h2m-hxy-u and
atmos.tas.tminavg-h2m-hxy-u all write variable_id = 'tas', so a tasmax
resume could adopt tas files as its checkpoint.

The same change fixes the split length for CMIP7 daily variables.
_canonical_frequency parses CMIP6 table names and falls back to 'mon' for
CMIP7 names, so split_years: auto resolved to 10 years for daily variables
instead of 5. The frequency is now taken from the compound name.

Attribute comparison is case-insensitive, because regions appear as both glb
and GLB.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.8%. Comparing base (a78c3fc) to head (a364821).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #743     +/-   ##
=======================================
+ Coverage   79.7%   79.8%   +0.1%     
=======================================
  Files         41      41             
  Lines       9173    9180      +7     
  Branches    1710    1711      +1     
=======================================
+ Hits        7315    7326     +11     
+ Misses      1524    1522      -2     
+ Partials     334     332      -2     

Flag	Coverage Δ
unit	79.8% <100.0%> (+0.1%)	⬆️

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