/**
 * Domain utilities for cross-subdomain cookie sharing.
 *
 * Uses a cookie-probe approach: tries setting a test cookie on progressively
 * shorter domain candidates until the browser accepts one. This is
 * self-maintaining — no hardcoded public suffix lists to keep up to date.
 * The browser itself is the authority on which domains accept cookies.
 */
/**
 * Extract the apex (registrable) domain for cookie sharing across subdomains.
 *
 * Walks up the hostname labels from shortest candidate to longest, probing
 * with a test cookie. The shortest domain the browser accepts is the apex.
 *
 * Returns null for localhost, IP addresses, single-label hosts, SSR, or
 * when no candidate domain accepts cookies (e.g. public suffix hosts like
 * vercel.app or github.io).
 *
 * Results are cached for the lifetime of the page.
 */
export declare function getApexDomain(): string | null;
/**
 * Reset the cached apex domain. Exposed for testing only.
 * @internal
 */
export declare function _resetApexDomainCache(): void;
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