perf: parallelise cmip7repack calls across split output files

[rbeucher]

Summary

When split_years produces N output chunks, cmip7repack was previously called sequentially (blocking subprocess) after each chunk was written — one at a time.

This PR dispatches all repack calls concurrently using ThreadPoolExecutor once all chunk files have been written.

Changes

• _write_single(): removed the inline _repack_cmip7_output + validate_cmip7_output calls.
• write() — multi-chunk path: writes all chunks sequentially, then repacks in parallel via ThreadPoolExecutor.map(), then validates sequentially (validation must follow repack).
• write() — single-file path (split off / fx variables): repack + validate called synchronously as before — no behaviour change.

Why threads are safe here

• Repack threads are only launched after all dask computations have been flushed and the netCDF4.Dataset file handles are closed, so there is no scheduler contention with dask.
• Each thread runs an independent subprocess.run(["cmip7repack", ...]) on a separate file — no shared state.

Impact

For a 165-year monthly run (17 × 10-year chunks), all 17 cmip7repack subprocesses now run concurrently instead of serially.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.8%. Comparing base (d0815eb) to head (74d2cc1).

Files with missing lines	Patch %	Lines
src/access_moppy/base.py	70.0%	1 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (70.0%) is below the target coverage (90.0%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #549     +/-   ##
=======================================
- Coverage   74.9%   74.8%   -0.0%     
=======================================
  Files         36      36             
  Lines       7156    7163      +7     
  Branches    1333    1336      +3     
=======================================
+ Hits        5357    5361      +4     
- Misses      1519    1520      +1     
- Partials     280     282      +2     

Flag	Coverage Δ
unit	74.8% <70.0%> (-<0.1%)	⬇️

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