# `RTL_LOCALES` and `RTL_LOCALE_LIST` — right-to-left locale set

## Purpose

`RTL_LOCALES` and `RTL_LOCALE_LIST` are the canonical inventory of
ISO 639-1 language codes whose primary script renders right-to-left.
Together they let any consumer (the locale detector, a UI component
deciding `dir="rtl"`, a logger annotating a span) determine text
direction with one O(1) lookup and zero parsing. Centralising the
list here means every package in the i18n family agrees on the same
seven RTL codes — no drift between formatters that might independently
hard-code Arabic + Hebrew while ignoring Persian or Urdu.

## Features

- 7 ISO 639-1 codes: `ar`, `he`, `fa`, `ur`, `ps`, `sd`, `yi`
- Two shapes — `ReadonlySet<string>` for membership checks,
  `readonly tuple` for iteration / serialisation
- Both shapes share the same data and the test suite asserts they
  agree on cardinality
- Tree-shake-friendly subpath at
  `@suaveplan/i18n-shared/constants/rtl-locales` for size-sensitive
  consumers

## Usage: Basic Example

```ts
import { RTL_LOCALES } from "@suaveplan/i18n-shared";

if (RTL_LOCALES.has("ar")) {
    document.documentElement.dir = "rtl";
}
```

## Usage: Advanced Example

```ts
import {
    RTL_LOCALE_LIST,
    RTL_LOCALES,
} from "@suaveplan/i18n-shared/constants/rtl-locales";

// Membership check — O(1) on the Set
function getDirection(locale: string): "ltr" | "rtl" {
    const language = locale.split("-")[0]?.toLowerCase() ?? "";
    return RTL_LOCALES.has(language) ? "rtl" : "ltr";
}

// Iteration — stable order from the tuple
for (const code of RTL_LOCALE_LIST) {
    console.log(`Loading RTL bundle for ${code}`);
}

// Serialisation — bundlers can inline the tuple as a JSON literal
const manifest = { rtl: [...RTL_LOCALE_LIST] };
```

## API Reference

### `RTL_LOCALES: ReadonlySet<string>`

Set of ISO 639-1 codes whose primary script is right-to-left. Use
this for O(1) membership checks.

### `RTL_LOCALE_LIST: readonly ["ar", "he", "fa", "ur", "ps", "sd", "yi"]`

Tuple form of the same data. Use this when iteration order matters,
when bundling into a manifest, or when the consumer wants to ship the
code list without the Set overhead.

## Implementation Notes

- The two exports are deliberately not derived from one another at
  module-init time — the Set takes the array path on construction and
  the tuple is a literal. A unit test asserts
  `RTL_LOCALE_LIST.length === RTL_LOCALES.size` so any future
  addition to one stays in sync with the other.
- The seven codes match the pre-split monolith exactly. Future
  additions (e.g. `dv` for Dhivehi) require updating both exports
  AND adding a corresponding `LOCALE_DESCRIPTORS` entry with
  `direction: "rtl"`.