Add GB↔GB-ZET exchange config for Shetland HVDC link

[sankalpsthakur]

Summary

• Adds config/exchanges/GB_GB-ZET.yaml for the Shetland HVDC Link between mainland Great Britain (GB) and the Shetland Islands (GB-ZET).
• Sets bidirectional capacity to 600 MW, matching the SSEN Transmission / Ofgem-approved Shetland HVDC Link rating.
• Addresses the filename feedback from #7534 (GB_GB-ZET.yaml, not GB_ZET.yaml).

Capacity source

• Ofgem Shetland HVDC Link Project Assessment Decision (2021): +/-320kV 600MW HVDC single circuit between Upper Kergord/Weisdale Voe (Shetland) and Noss Head (Caithness)
  • https://www.ofgem.gov.uk/sites/default/files/2021-11/Shetland%20Link%20Project%20Assessment%20Decision%20FINAL.pdf
• SSEN Transmission project page: https://www.ssen-transmission.co.uk/projects/project-map/shetland/

Notes

• No live exchange parser is included: Elexon/BMRS interconnector feeds cover international links (FR, BE, NL, NO, IE), not this domestic SSEN transmission asset. This matches other capacity-only exchange configs in the repo (e.g. FR_JE, AX_SE, IT_MT).
• Lonlat is placed roughly midway on the subsea route; rotation points toward GB-ZET for positive GB→GB-ZET flow.

Fixes #7269

Test plan

• Config loads as GB->GB-ZET via EXCHANGES_CONFIG / CONFIG_MODEL
• uv run python -m unittest tests.test_exchanges_json -v
• Visual check of exchange arrow on map (staging / local web if available)

AI/LLM disclosure

• AI coding tools (including Cursor agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.