Add alternative biomass allocation criterion to scenario with exogenous biomass limit

[merfort]

Purpose of this PR

[This PR was created with help of AI and manually reviewed and tested afterwards]

Quick-fix for cm_maxProdBiolc: instead of distributing the whole global lignocellulosic biomass potential to regions purely by equal marginal supply costs, a slice of up to 25 EJ/yr is now allocated by hardcoded shares of 2020 total agricultural crop production (from a recent coupled REMIND-MAgPIE NPi run). The remainder is still distributed via the marginal-cost inversion. The slice is scaled down proportionally when the total bound is smaller than 25 EJ/yr, and the
implementation works for both the H12 and EU21 region resolutions (values are hardcoded at the level of the 12 H12 region groups; EUR/NEU are split to their EU21 sub-regions by 2020 MAgPIE lignocellulosic biomass production from a look-up table, as this is already available in the read-in input data. Global totals are conserved.

This PR is the near-term quick-fix for this issue. There you also find plots showing how the new bound works. Medium-term we want to implement a cleaner solution.

Type of change

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Parts concerned

• ☑️ GAMS Code
• ◻️ R-scripts
• ☑️ Documentation (GAMS incode documentation, comments, tutorials)
• ◻️ Input data / CES parameters
• ◻️ Tests, CI/CD (continuous integration/deployment)
• ◻️ Configuration (switches in main.gms, default.cfg, and scenario_config*.csv files)
• ◻️ Other (please give a description)

Impact

• ◻️ Bug fix
• ◻️ Refactoring
• ☑️ New feature
• ◻️ Change of parameter values or input data (including CES parameters)
• ☑️ Minor change (default scenarios show only small differences)
• ◻️ Fundamental change of results of default scenarios

Checklist

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In the end all checkboxes must be ticked before you can merge.

• I executed the automated model tests (make test) after my final commit and all tests pass (FAIL 0)
• I adjusted the reporting in remind2 if and where it was needed
• I adjusted the madrat packages (mrremind and other packages involved) for input data generation if and where it was needed
• My code follows the coding etiquette
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• I updated the CHANGELOG.md correctly (added, changed, fixed, removed, input data/calibration)

Further information (optional)

• Runs with these changes are here:
  /p/tmp/merfort/REMIND_bio_bound_by_production/remind/output

• Comparison of results (what changes by this PR?):

SSP2−NPi2025 - original NPi run
SSP2−NPi2025.1 - NPi run with updated bounds
SSP2−PkBudg750_100EJ_orig - original PkBudg750 run
SSP2−PkBudg750_100EJ - PkBudg750 run with updated bounds

The full compare scenarios PDF can be found here:
compScen-NPi2025-PkBudg750_old-new-2026-07-03_14.29.31-H12.pdf

For more information check the development issue.

[robertpietzcker]

The results look good, and from a brief reading of the code it seems reasonable.