/**
 * Detect a top-of-file `"use client"` directive in RAW source text (including
 * untranspiled TSX, which the acorn-based detectors can't parse).
 *
 * Used during build-time auto-discovery, where we must scan first-party
 * `.tsx/.ts/.jsx/.js` files on disk to decide which directive-only client
 * modules to emit to `dist/client`. We can't run the JSX/TS-aware Rollup
 * parser there (no plugin context), so we apply React's own structural
 * contract directly: a file-level `"use client"` must be a string-literal
 * statement at the very top of the module, before any real code.
 *
 * This is deliberately stricter than the naive `code.match(/client/)`
 * substring check (which flags any module mentioning the word "client"):
 *   - leading whitespace, line (`//`) and block (`/* *\/`) comments are
 *     skipped (they're allowed above a directive),
 *   - a leading `"use strict"` prologue is tolerated (real libraries ship
 *     `"use strict"; "use client";`),
 *   - the directive must then be the first actual statement.
 * Anything else (an import, a variable, the word "client" in a comment or
 * identifier) yields false.
 */
export declare function sourceHasTopLevelClientDirective(source: string): boolean;
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