fix(search): sort a document that has no title in the reader’s language

[ddeboer]

Sorting a collection by a localized text field returned it unsorted whenever the
documents were not titled in the reader’s language.

A sortable text field wrote a sort key only for the languages a document
actually carried: a work with an untagged title got name_sort_und and nothing
else, one titled in English got name_sort_en and nothing else. A sort, unlike
a search, names a single key – the query compiler picks the active locale’s.
So orderBy: {field: NAME, direction: DESC} from a Dutch client compiled to
name_sort_nl:desc, on which every document without a Dutch title carries the
empty string. They all tie, and Typesense hands them back in relevance order –
which is exactly what an unsorted result looks like.

Each locale’s sort key now falls back to the document’s first value in locales
order, so a sort key is present whenever the field holds any value in a declared
locale. A collection of untagged titles sorts by title in a Dutch request rather
than not at all; a work titled only in English sorts under its English title
next to the Dutch ones. Ordering across languages is approximate by nature, and
a total order over the titles a reader actually sees beats a partial one over
the tagged few.

The search companions deliberately keep no fallback: a query fans out over every
locale key at once, so a document is already found whichever language the reader
asks in, and copying the value would only inflate the index.

Reported downstream at https://codeberg.org/limburg/lol/issues/158, on a
register whose CreativeWork titles are predominantly untagged.

Changes

• packages/search/src/project.ts – applyText computes the values per
  declared locale once, and fills each locale’s sort key from the first
  non-empty bucket when its own is empty. Search keys are unchanged.
• packages/search/test/project.test.ts – a document titled only in English
  sorts under that title in every declared locale, while its search keys stay
  per-locale.
• packages/search-pipeline/test/registry-extraction.integration.test.ts – the
  projected dataset now also carries label_sort_und, the fallback for a
  request that states no language.
• docs/reference/search.md – the Locales section states the fallback, why a
  sort needs one where a search does not, and that a sort field is absent only
  when the field is.