Fix heat pump constraints - #787

Changes proposed in this Pull Request

This PR fixes heating/cooling coupling constraints for air-source and ground-source heat pumps.

The previous implementation selected cooling Links using -cooling, although the network builder uses -cool. It also mixed the Link and Link-ext dimensions, introducing a Cartesian product instead of one operating constraint per heat pump pair and snapshot.

Changes include:

• Pair heating and cooling Links explicitly by name, independently of their ordering.
• Align dispatch, COP profiles and nominal capacities on the same Link dimension.
• Preserve the existing capacity equality and shared heating/cooling output limit.
• Support static and time-varying COPs, selected model snapshots, and fixed capacities, including those frozen during myopic optimisation.
• Validate missing counterparts and inconsistent fixed capacities.

Changes are confined to workflow/scripts/opts/sector.py and the branch builds on defer_sector_demand (see #786 ), which in turn builds on improve_workflow (see #785 ).

Model size and solver runtime observed on the full US sector-coupled model with 134 clusters and 168h resolution:

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.