test: cover the twilight API with astral-generated fixture data

[ceeK]

Summary

The six twilight properties — civilSunrise/Sunset, nauticalSunrise/Sunset, astronomicalSunrise/Sunset — had no coverage; only official sunrise/sunset were tested, leaving three quarters of the public date API unverified.

This adds the six columns to all 237 cities in CorrectResults.json and six parameterized tests mirroring the existing ones, at the same 5-minute accuracy.

Where the expected values come from

Generated with the astral package — an independent NOAA-based implementation — never from Solar itself, so the tests verify the zenith constants are astronomically right rather than merely self-consistent.

The source was validated before being trusted: regenerating the existing sunrise/sunset columns with astral reproduced the committed values to a max of 173s (median 29s), comfortably inside the 5-minute tolerance.

Two things worth a reviewer's attention

The fixture stores events on the local solar day, not the UTC day. For far-eastern/western cities (Wellington, Mexico City) the committed timestamps sit on the neighbouring UTC date, matching Solar's shouldBeYesterday/shouldBeTomorrow shift. I reproduced this by picking the occurrence nearest local mean noon, derived from longitude alone, rather than copying Solar's own heuristic — otherwise a bug in that heuristic would be baked into the fixture meant to test it.

No nulls were needed. All 237 capitals get all three twilights on 2017-02-09, so City uses non-optional Dates consistent with the existing fields. Adding a polar city later would need optionals; it currently fatalErrors with a message naming the missing key.

Verification

• swift test: 20 tests pass, including 1,422 new city assertions — no extra tolerance needed, even at Nuuk and Reykjavík.
• The JSON serialiser was proven byte-identical to the committed fixture before writing, so the diff is purely additive: 0 pre-existing field values changed and the city names repaired in fix(tests): repair mojibake'd city names in CorrectResults.json #83 are byte-for-byte intact.

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