add ISO 3166-1 source verification test

[gronke]

This is a proposal, offered as working code so the discussion has something concrete; happy to adjust or drop any part of it.

The country! table in lib.rs is maintained by hand, and it is correct: every (alpha2, alpha3, numeric) triple matches the ISO 3166-1 data of the Debian iso-codes project (the transcription of the official ISO publication that most distributions ship), with XK as the deliberate user-assigned extra.
What the crate cannot currently prove is that this stays true: ISO amends the standard occasionally, and nothing would notice a divergence today.

This PR adds that proof:

• tests/iso_oracle.rs compares the full table against a committed snapshot of iso-codes' iso_3166-1.json, failing on any difference in either direction, with a documented exception list (XK).
• The snapshot stays repository-only: the package include list ships tests/*.rs alone, so the published crate remains exactly Apache-2.0 OR MIT. For the repository itself the fixture's terms are handled the way LICENSE-UNICODE already handles the CLDR data: tests/fixtures/README.md carries the source and copyright notice, and the LGPL-2.1 text sits next to the file it covers, scoped to it alone. If you would rather keep the repository entirely free of LGPL material, the same check can instead read /usr/share/iso-codes/json/iso_3166-1.json from the CI runner after installing the iso-codes package (skipping gracefully where it is absent); say the word and I rework it that way.
• The README gains a short Data provenance section stating the check.

No CI changes required: the existing workflow's cargo test --all-features picks the test up.

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[mikelodder7]

hmm, when I do a package and extract test which is what crates.io does, I get

error: couldn't read `tests/fixtures/iso_3166-1.json`: No such file or directory
  --> tests/iso_oracle.rs:31:30

Anyone running the test suite from the crates.io package — which is exactly what distro packagers do, ironically the Debian folks this fixture comes from — get a compile failure. cargo package's verify step doesn't catch it because it only builds the lib. Current main doesn't have this problem, so it's a genuine regression introduced by this PR.

We could just exclude iso_oracle.rs from the package which seems simple enough. Is that something you'd be willing to do?