i18n: add Central Kurdish (Sorani) as an interface language

2,670 entries in ckb.tsv, written in logical order, with the same
(context, source) key set as ar.tsv. Registered with rtl: true and
plural_one_other (CLDR ckb: one, other); Sorani keeps the noun singular
after a numeral, so the 13 @plural rows usually read the same in both
forms.

RTL reuses the Arabic path: the catalog is put into display order by
bidi::display_rtl when it loads, so Kurdish has the same four limits
docs/localization.md lists for Arabic. No new layout or bidi code.

DATE_LANGUAGES gains Kurdish month and weekday names, because
date_languages_are_the_interface_languages keeps both registries in
step.

ku-* locales are not mapped to ckb on purpose: ku is the macrolanguage
and means Kurmanji in Latin script. Only ckb* tags resolve.

ATTRIBUTION.toml

@@ -208,6 +208,17 @@ kind = "translation"
 usage = "Arabic UI strings, compiled in by crates/ui-egui/src/i18n/mod.rs"
 sha256 = "889a0d0a9ea53ca0a818c2ba672c053c232d2cca140f8d63127353e6f36505fe"
 
+[[asset]]
+path = "crates/ui-egui/src/i18n/ckb.tsv"
+title = "Central Kurdish (Sorani) interface translation catalog"
+author = "PdfCraft contributors"
+source = "Contributor-original: clean-room translations of PdfCraft's English UI labels, no proprietary localisation resources"
+licence = "MIT OR Apache-2.0"
+licence_file = "LICENSE-MIT"
+kind = "translation"
+usage = "Central Kurdish UI strings, compiled in by crates/ui-egui/src/i18n/mod.rs"
+sha256 = "2479d8f164612c4e8d3944df9a6b2ae345be0b26ee87511b17a4a56bc41988d3"
+
 [[asset]]
 path = "crates/ui-egui/src/i18n/tr.tsv"
 title = "Turkish interface translation catalog"