/**
 * Creates a reactive side-effect that runs immediately and re-runs whenever
 * its dependencies change.
 *
 * - Runs `fn` immediately on creation.
 * - Automatically tracks any atoms/stores read inside `fn` as dependencies.
 * - Re-runs `fn` whenever any dependency changes (scheduled via the effect scheduler).
 * - If `fn` returns a cleanup function, that cleanup runs before the next execution
 *   and when the effect is disposed.
 * - Returns a `dispose` function — call it to stop the effect and run the final cleanup.
 * - `fn` is marked `__isEffect__ = true` so the reactive runner knows it supports cleanup.
 * - Must NOT be called inside a template function (restricted context guard applies
 *   via `createReactiveRunner`).
 *
 * @param fn - The side-effect function to run. May return a cleanup function that
 *             is called before the next run and on disposal.
 * @returns A `dispose` function that stops the effect and runs the final cleanup.
 *
 * @example
 * const count = atom(0);
 *
 * const stop = effect(() => {
 *   console.log("count is", count()); // runs immediately, then on every change
 *   return () => console.log("cleanup"); // runs before next execution
 * });
 *
 * count.set(1); // logs "cleanup", then "count is 1"
 * stop();        // dispose — runs final cleanup
 */
export declare function effect(fn: () => (() => void) | undefined | void): () => void;
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