[Feature] Reactive-Streams interop (Akka-Streams subset interop)

[pathosDev]

Size / Priority

• Size: S (depends on [Feature] Akka-Streams DSL subset (SourceQueue, MergeHub, BroadcastHub) #147 — Akka-Streams DSL)
• Reference: Reactive Streams Spec, Node stream/web.

Rationale

If #147 (Streams DSL subset) ships, interop with the Reactive Streams spec makes our hubs interoperable with:

• Node's stream/web (ReadableStream / WritableStream).
• Bun's ReadableStream (same spec).
• Any RxJS observable (via from(publisher)).
• Any @nodejs/streams-based code.

The Spec is small (4 interfaces). Adapters are ~200 lines.

Depends on #147: without the Streams DSL, there's nothing to wrap.

Design sketch

// src/streams/interop/ReactiveStreams.ts (new)

import type { Publisher, Subscriber, Subscription } from './reactive-streams-types.js';
import type { SourceQueue, BroadcastHubSource } from '../Streams.js';

/** Wrap our SourceQueue as a Reactive Streams Publisher. */
export function toPublisher<T>(queue: SourceQueue<T>): Publisher<T>;

/** Wrap a Reactive Streams Publisher as our Source. */
export function fromPublisher<T>(publisher: Publisher<T>): SourceQueue<T>;

/** Web Streams interop. */
export function toReadableStream<T>(queue: SourceQueue<T>): ReadableStream<T>;
export function fromReadableStream<T>(stream: ReadableStream<T>): SourceQueue<T>;

Integration

• Depends on [Feature] Akka-Streams DSL subset (SourceQueue, MergeHub, BroadcastHub) #147's Streams DSL.
• Backpressure preserved: Reactive Streams request(n) maps to our offer semantics.

Out of scope / non-goals

• RxJS-specific — RxJS observables are interop-compatible via Publisher; no special wrapper.

Open design questions

1. Buffer policy when bridging RS Publisher with infinite demand vs our bounded SourceQueue. Recommend: respect our queue's overflow strategy.
2. Async error propagation: both sides have error channels; map cleanly.

Test plan

1. Pipe Node Readable → fromReadableStream → Source → consumer; no data loss.
2. Pipe our Source → toReadableStream → Node Writable; works.
3. Backpressure: slow consumer → publisher slows.
4. Error propagation in both directions.

Acceptance criteria

• toPublisher / fromPublisher adapters.
• toReadableStream / fromReadableStream for Web Streams.
• Backpressure preserved.
• Documentation.
• Test suite.
• CHANGELOG entry.
• Block until [Feature] Akka-Streams DSL subset (SourceQueue, MergeHub, BroadcastHub) #147 lands.