Adjust .ssp.transport

[khaeru]

This PR implements changes to .ssp.transport.process_df(…, method=METHOD.C) requested today:

• Reduce Emissions|CO2|Energy|Demand|Transportation by the amount computed for Emissions|CO2|Energy|Demand|Bunkers|International Aviation.
• Ensure the reduction in Emissions|…|Transportation for region="World" is equal to added values for Emissions|…|Bunkers|International Aviation.
  • This meant disabling the previously-specified method where the share/portion of AIR transport in total transport final energy consumption of fossil fuels was used to reduce the global total.
• Update variables such as the following, in particular the third which was not previously included:

  Emissions|BC
  Emissions|BC|Energy
  Emissions|BC|Energy and Industrial Processes
  Emissions|BC|Energy|Demand
  

• In contrast for the first item, hold values for Emissions|{e}|Energy|Demand|Transportation fixed, where e ≠ "CO2".
• Overwrite, rather than increment, any existing values for Emissions|{e}|Energy|Demand|Bunkers|International Aviation.
  • Adjust ‘higher’ aggregates for the difference between those existing values and the replacements.
• Also write/adjust final energy [or all] values for year=2020.
• Update implied emission factors in data/transport/emi-intensity.csv.
• Compute final energy values (thus derived emissions values) for n=World by sum across other regions, rather than by adjusting existing n=World values.

The commits are also reproduced on ssp-dev.

How to review

• Read the diff.
• Note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.1%. Comparing base (e817c34) to head (2c0c7ad).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #485   +/-   ##
=====================================
  Coverage   74.1%   74.1%           
=====================================
  Files        318     318           
  Lines      25560   25568    +8     
=====================================
+ Hits       18952   18966   +14     
+ Misses      6608    6602    -6     

Files with missing lines	Coverage Δ
message_ix_models/project/ssp/transport.py	100.0% <100.0%> (+2.9%)	⬆️
...sage_ix_models/tests/project/ssp/test_transport.py	99.2% <100.0%> (+<0.1%)	⬆️

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[khaeru]

Merging despite the job macos-latest-py3.14-upstream-main not completing; this job is known to be flaky and has stalled multiple times on this PR.