[akesson]

Summary

With both en and en-gb built in, L10n::langneg("en-GB") returned L10n::En. Negotiation compared only the language subtag and took the first locale that matched.

Now each Accept-Language entry, in q order, picks the closest locale:

1. the exact locale (en-GB → en-gb);
2. the same region with no conflicting script (sr-Latn-RS → sr-RS);
3. the generic locale (en-AU → en, even if en-gb comes first);
4. another region (en-AU → en-gb when there's no en);
5. last, a locale with a conflicting script (sr-Latn → sr-Cyrl only when no other sr exists).

Ties go to the locale without a script the request didn't ask for (en → en, not en-Latn). The header's own order still comes first. In de-AT, en-GB;q=0.5, a plain de beats an exact en-gb, because the user prefers German.

Entries with q=0 ("not acceptable") or an invalid q are now skipped. Before, a q=0 entry could still be picked, and an invalid weight counted as q=1.

Known limit: scripts aren't inferred from regions, so zh-TW with both zh-Hans and zh-Hant available takes the first one. Fixing that needs likely-subtags data (icu_locale). That would add 18 crates and about 17 KB to langneg builds, so it's left for a separate decision. The docs and a test state this limit.

Before and after, with en, en-gb and fr:

Header	Before	After
en-GB	En	EnGb
en-US	En	En
fr-CA	Fr	Fr

Changes

• prelude::best_match ranks each candidate once with a plain rank function. Both the generated L10n::langneg (through negotiate_languages) and L10nLanguageVec::langneg use it, so they can't disagree, and neither allocates per call any more.
• negotiate_languages no longer requires PartialEq on the language type.
• negotiate_languages had no tests; there are now tests for each tier and for header order.
• The languages guide describes the new behaviour with assert_eq! examples. docs-check runs them, now against an added en-gb locale.
• CHANGELOG entry under Fixed.

This changes which locale some requests get, but only towards the exact or more generic match, so I've listed it as a fix rather than a breaking change.