/**
 * STREAM ACCUMULATION
 * ===================
 *
 * The server streams text a piece at a time.
 *
 * It used to send the whole message again on every token, which is O(N²) bytes for an
 * N-token reply — slow everywhere, and fatal on a Link websocket, where the answer queues
 * ahead of the connection's own heartbeat until this SDK closes it mid-sentence. A
 * streamed value now arrives as one of two shapes, and never both:
 *
 *     { messageId, message: "Good day to you", completed }   // the whole value: replace
 *     { messageId, delta: " to you", completed: false }      // what was added: append
 *
 * Deltas are also sent bare: no metadata block, since it is the same on every frame of a
 * message and several times the size of the few characters a delta carries. The frame that
 * opens a message brings it, and the one that finishes it brings it again.
 *
 * Callers should not have to care about any of that. This puts the message back together,
 * so `payload.message` is the whole message so far exactly as it always was, restores the
 * metadata onto every event, and keeps `payload.delta` for anyone who would rather append
 * than re-render.
 *
 * Whole values arrive for the last event of a message and for anything replaying after a
 * reconnect, and they replace rather than extend. That is what makes a reconnect cheap and
 * a dropped delta harmless, and it is handled here so no caller has to know about it.
 */
/**
 * Rebuilds whole values from a stream of pieces.
 *
 * Stateful, and one per stream: it holds what every message the stream is still writing
 * has said so far, so a turn and a progress stream never see each other's.
 */
export declare function createStreamAccumulator(): (payload: unknown) => unknown;