iOS: OutputStream.write return value 0 treated as fatal in bound stream writers

Summary

The writeAll pattern used in bound-stream-pair writers treats OutputStream.write(_:maxLength:) returning 0 as a fatal error. Apple's documentation defines a 0 return as "a fixed-length stream has reached its capacity." It is unclear whether a bound stream pair (from Stream.getBoundStreams(withBufferSize:)) qualifies as a "fixed-length stream," meaning write() could return 0 when the internal buffer is full rather than blocking.

If 0 is returned as a backpressure signal, the current code silently aborts the write, closes the stream, and leaves URLSession waiting for Content-Length bytes that never arrive — causing the upload to hang.

Locations

1. RequestBody.makePipedFileSliceStream — ios/Sources/GutenbergKitHTTP/RequestBody.swift (~line 236)
2. DefaultMediaUploader.writeAll — ios/Sources/GutenbergKit/Sources/Media/MediaUploadServer.swift (introduced in #419, same pattern)

Both use:

let result = output.write(base.advanced(by: written), maxLength: ...)
if result <= 0 { return }

Apple documentation

• OutputStream.write(_:maxLength:): returns 0 for "a fixed-length stream that has reached its capacity"
• CFWriteStreamWrite: "If the stream is not full, this call blocks until at least one byte is written"
• Neither document explicitly states whether bound stream pairs block or return 0 when the buffer is full

Suggested fix

Distinguish 0 (retry) from -1 (error):

if result == -1 { return false }
if result == 0 {
    Thread.sleep(forTimeInterval: 0.001)
    continue
}

Risk

Low probability in practice — uploads would need to outpace URLSession's read rate long enough to fill the 65KB buffer. More likely with large files on slow connections.

Start with RequestBody.makePipedFileSliceStream in ios/Sources/GutenbergKitHTTP/RequestBody.swift and DefaultMediaUploader.writeAll in ios/Sources/GutenbergKit/Sources/Media/MediaUploadServer.swift. Read how each handles OutputStream.write(_:maxLength:) results, then verify that a zero return does not abort an upload while a negative return still does. Done means both bound-stream writers handle the documented return cases consistently without leaving URLSession waiting for bytes.