fix(rest): localize the canonical PLURAL /meta spelling, not just the singular (#6349) (#7372)

`translateMetaItem` / `translateMetaItems` decide whether a metadata type is
localizable by asking `isTranslatableMetaType`, which reads
`TRANSLATABLE_METADATA_TYPES` — a set DERIVED (#3786) from
`METADATA_DOCUMENT_TRANSLATORS`' keys, and those keys are singular-only
(view / action / object / app / dashboard / page), matching
`translateMetadataDocument`'s documented "Canonical metadata type string".

The three `/meta` read handlers handed those helpers the RAW `:type` path
segment. Prime Directive #3 makes PLURAL the canonical REST spelling, so a
caller following the documentation missed the set, the predicate answered
false, and the whole localization was skipped — same route, same document,
same Accept-Language, two different answers.

Normalized in the two helpers (the issue's option 2) rather than at the five
call sites, for the reason #6241 proved: a normalization the callers own is
one a later caller forgets. `metaTypeSingular` leaves an unmapped type
untouched, so the translatable SET is unchanged — only the spellings that
reach it.


Claude-Session: https://claude.ai/code/session_0158ZQo7LiHSxGWpYKuPq1wu

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-help

.changeset/meta-plural-spelling-i18n.md

@@ -0,0 +1,18 @@
+---
+'@objectstack/rest': patch
+---
+
+`/meta` reads localize the canonical PLURAL spelling, not just the singular
+
+The three metadata read handlers (`GET /meta/:type`, `GET /meta/:type/:name`,
+`GET /meta/:type/:section/:name`) handed the raw `:type` path segment to the
+translate helpers, whose "does this type translate" predicate reads a set derived
+from singular-only translator keys (`view` / `action` / `object` / `app` /
+`dashboard` / `page`). Prime Directive #3 makes plural the canonical REST
+spelling, so a caller following the documentation received unlocalized
+labels/descriptions/navigation while the singular spelling of the same route
+returned the translated document.
+
+`translateMetaItem` / `translateMetaItems` now fold the spelling to the canonical
+singular before asking, so both spellings answer the same localized body. The set
+of translatable types is unchanged — only which spellings reach it.