/**
 * URL-backed debug flags — the single seam hosts use to mirror an engine debug surface (the scene
 * editor, the devtools overlay) into the page query string. Mirroring means the URL is the honest
 * record of what is open: share it to reproduce the state, and strip the param (then reload) to get
 * a clean game with the surface gone. All functions no-op safely off the main browser thread.
 */
/** The current query value of `param`, or null when absent (or no DOM). */
export declare function readUrlParam(param: string): string | null;
/** True when `param` is present and not an explicit off value (`0`/`false`/`off`/`no`). */
export declare function readUrlFlag(param: string): boolean;
/**
 * Sets `param` to `value`, or removes it when `value` is null, rewriting the URL in place with
 * `history.replaceState` so the rest of the query and the hash survive and no history entry is
 * pushed. No-ops without a DOM, and skips the write when the URL already matches.
 */
export declare function writeUrlParam(param: string, value: string | null): void;
/**
 * Fires `listener` whenever the query string may have changed out from under a flag — browser
 * back/forward (`popstate`). Manual address-bar edits reload the page, so initial reads cover
 * those; this keeps in-app state honest across history navigation. Returns an unsubscribe.
 */
export declare function subscribeUrlChange(listener: () => void): () => void;