Implement Streams API foundation: ReadableStream, WritableStream, TransformStream #182

Goal#

Implement the WHATWG Streams API foundation: ReadableStream, WritableStream, and TransformStream as JS-exposed classes in we-js, with their default controllers, default readers and writers, and the queuing strategies and state machines required by later issues.

Scope#

• ReadableStream
  • Constructor: new ReadableStream(underlyingSource?, strategy?)
  • underlyingSource callbacks: start, pull, cancel, type (undefined → default, 'bytes' deferred to the next issue)
  • ReadableStreamDefaultController: enqueue(chunk), close(), error(e), desiredSize
  • ReadableStreamDefaultReader: getReader(), read() → Promise<{ value, done }>, releaseLock(), closed, cancel(reason)
  • State: readable / closed / errored
  • locked getter
  • Async iterator (@@asyncIterator) on ReadableStream so for await (const chunk of stream) works
• WritableStream
  • Constructor: new WritableStream(underlyingSink?, strategy?)
  • underlyingSink callbacks: start, write, close, abort, type (reserved)
  • WritableStreamDefaultController: error(e), signal
  • WritableStreamDefaultWriter: getWriter(), write(chunk), close(), abort(reason), releaseLock(), desiredSize, ready, closed
  • State: writable / closing / closed / erroring / errored
• TransformStream
  • Constructor: new TransformStream(transformer?, writableStrategy?, readableStrategy?)
  • transformer callbacks: start, transform, flush
  • TransformStreamDefaultController: enqueue, terminate, error, desiredSize
  • Exposes a paired readable and writable
• Queuing strategies
  • CountQueuingStrategy({ highWaterMark }) with .size = () => 1
  • ByteLengthQueuingStrategy({ highWaterMark }) with .size = (chunk) => chunk.byteLength
• Internal queue with desiredSize = highWaterMark - currentSize semantics
• Microtask scheduling: pulls and writes run via the existing microtask queue from Phase 10

Acceptance Criteria#

• new ReadableStream(...).getReader().read() returns a thenable that resolves to { value, done } matching the WHATWG semantics
• Backpressure: controller.desiredSize decreases as chunks are enqueued, and pull() is invoked when desiredSize > 0 and the queue is below the high water mark
• Errored streams reject pending reads/writes and subsequent read()/write() calls
• writable.getWriter().write(chunk) returns a promise that resolves when the underlying sink accepts the chunk (respecting backpressure via ready)
• TransformStream paired streams chain backpressure correctly (writable ready depends on readable buffer level)
• Async iteration over a ReadableStream yields enqueued chunks and terminates on close
• Unit tests covering: default reader read/close/error/cancel, default writer write/close/abort, queueing strategies, transform stream identity, error propagation, locking, async iteration
• Conventions
  • cargo fmt --all --check
  • cargo clippy --workspace -- -D warnings
  • cargo test --workspace
  • No external crate dependencies; unsafe only inside js if strictly needed for GC integration