Apply seasonal line ratings to s_max_pu without modifying s_nom

[MaykThewessen]

Addresses the review feedback on PyPSA#1694.

What changed

The previous Lines.apply_seasonal_rating set s_nom to the winter envelope (max(summer, winter)) in place and encoded the seasonal variation as a derived ratio. The reviewer's key requirement was:

The important bit is that s_nom is not changed inplace by applying the seasonal ratings. They should only apply the scaling to s_max_pu.

The method now:

• Treats the summer / winter columns as absolute MVA ratings — the form TSOs actually publish. The JAO / Core flow-based domain files state a maximum permissible current per season and voltage level, which together define an absolute MVA limit for a line. (This also answers the reviewer's question: TSOs report ratings in absolute capacity terms, not per-unit load factors.)
• Converts those absolute ratings into per-unit factors relative to each line's existing s_nom: s_max_pu = rating(season) / s_nom per snapshot.
• Never modifies s_nom. It stays the nominal rating the network was built with, so capacity/investment logic is unaffected.
• Validates that referenced lines have a strictly positive s_nom.
• Still composes with pre-existing static/dynamic s_max_pu (e.g. N-1 margins) via compose=True.

Other review items addressed

• Tests reworked around a seasonal_network fixture with parametrized season cases, and renamed to test_seasonal_line_ratings.py. Includes an explicit test_s_nom_never_changes_for_any_input.
• Docs page simplified: clarifies that inputs are absolute MVA and that s_nom is preserved; dropped the "out of scope" section and trimmed the standards language.
• Added a release note and an mkdocs nav entry.

Verification

• pytest test/test_seasonal_line_ratings.py → 17 passed.
• Method docstring doctest passes.
• ruff check / ruff format --check clean.

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