/**
 * `--sourcemap=linked` appends a `sourceMappingURL` comment, so a browser with devtools open
 * fetches that file on every load. When it is not in the served root the request fails, and the
 * failure surfaces in exactly the session where someone is reading the console carefully for
 * something else — it cost the reporter a wrong hypothesis while chasing a slow load
 * (tosijs-ui#103). Before that it was worse: the dev server's SPA fallback answered with the
 * HTML shell, so devtools received a web page where a JSON map should be (fixed separately,
 * #116).
 *
 * Checks what is SERVED rather than what was built. Those are the same directory by default and
 * different when `bundleOutDir` is set, and the copy across is best-effort by design — a missing
 * map must never fail a build. So this verifies the end state instead of trusting the step.
 *
 * Returns the message, or `null` when there is nothing to say. A pure function taking its own
 * `exists` so it can be tested without a filesystem: the call site lives in a branch this repo's
 * own build does not take (its bundle is built by `bin/dev.ts`, not by `buildSite`), which is
 * exactly why the bug reached an adopter and not us.
 */
export declare function sourcemapWarning(bundleJs: string, publicDir: string, exists: (p: string) => boolean): string | null;