NDC CO2 Price: do not allow co2 price to drop after first target year

[fschreyer]

Purpose of this PR

Setting that co2 price cannot drop below co2 price of first target year switched on by default.

Type of change

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Do not delete any lines. This makes it easier to understand which areas are affected by your changes and which are not.

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/schreyer/Modeling/remind/Dev1/output/NDC_NonDecreasing_2026-07-01_22.13.24
• Comparison of results (what changes by this PR?):

[lea-hayez]

@fschreyer could you share the compScen link?

[fschreyer]

These are the changes to the NDC:

/p/tmp/schreyer/Modeling/remind/Dev1/compScen-NDC_NonDecreasing_v3-2026-07-02_15.14.03-H12-to2050.pdf