i18n: a locale per app, catalogs of messages, mirrored stacks

i18n::I18n is a gpui global holding the shown locale and one catalog of
messages per locale. text fills each {name} from its arguments; a
missing key, argument or catalog fails, and every catalog holds the
first one's keys. Locale writes numbers with its separators and dates
in CLDR's short numeric form; Hebrew replaces Arabic, whose Egyptian
digits Ely does not write. Direction::row mirrors a stack and
Direction::align sets text against the reading side. The gallery's
chapter 40 shows the messages in seven locales, every locale's number
and date, and a file list that mirrors under Hebrew.

AGENTS.md

@@ -190,6 +190,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A pane the window's blur shows through is `shell::Vibrancy`, filled with the palette's `glass`, in a window opened `WindowBackgroundAppearance::Blurred`: vibrancy on macOS, acrylic on Windows. gpui 0.2.2 has no Mica.
 - The chart's hues go by `theme::HUE_NAMES`, for an event's color and a label's alike.
 - Text whose every character counts, encoded data and keys, goes through `typography::literal`: JetBrains Mono draws `==` and `->` as ligatures (`devtools::Encoder`).
+- Messages go through `i18n::I18n`, a global the app sets: `cx.global::<I18n>().text(key, args)` fills each `{name}`. Every catalog holds the first one's keys; a missing key, argument or catalog fails. A right-to-left locale mirrors a stack with `Direction::row` and sets its text with `Direction::align`; CoreText orders mixed Hebrew and Latin runs on macOS.
 
 ## Decisions