fix: Avoid duplication of regional co2 sequestration potential in myopic optimization

[lindnemi]

Closes #2368.

Changes proposed in this Pull Request

AI-assisted

Replace sequestration Stores with fixed-capacity, negative-dispatch Generators. Regional sequestration potentials are enforced through e_sum_min, without accumulating additional potential across myopic planning horizons.

• Set tracking Generators’ lifetime to infinity so existing brownfield processing drops previous copies.
• Use finite, fixed nominal capacities for sequestration Links and Generators, to avoid additional investment variables.
• Move sequestration costs from Links to Generators, using a negative marginal cost to charge for negative dispatch.
• Update the European sequestration-limit check to detect Generators instead of Stores.
• Document the Generator-based regional limit.

Workflow comparison

Compared results/master with results/co2-seq-final for 2030, 2040 and 2050. Saved run configurations are identical except for the output prefix.

The fix keeps the summed regional sequestration allowance at 649.1 Mt/year in every horizon. In master, inherited Stores inflate it to 1,298.2 Mt/year in 2040 and 1,947.3 Mt/year in 2050.

Metric	2040: master → fixed	2050: master → fixed
Actual sequestration	180 → 180 Mt	250 → 250 Mt
Summed CO₂ pipeline capacity	36,048 → 44,655 t/h (+23.9%)	48,359 → 62,488 t/h (+29.2%)
CO₂ pipeline capital-cost contribution	€2.849 → €3.507 bn (+23.1%)	€3.947 → €5.178 bn (+31.2%)
Total system costs	+0.089%	+0.175%

Removing duplicated regional allowances relocates sequestration toward regions with spare potential. For example, DK2 sequestration increases from 14.2 to 80.5 Mt in 2040 and 17.3 to 131.1 Mt in 2050, explaining the higher transport requirements.

In 2030, sequestration remains negligible in both runs (748 → 275 tonnes), and total system costs change by +0.022%.

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.md.
• The description is human-written and any AI-generated content is marked.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.md files.