- Closes#712.
- Replace synchronous `write()` calls in
`BidirectionalRelay` with non-blocking I/O
and `DispatchSourceWrite` backpressure
handling. Under concurrent vsock proxy load,
a single blocked write on the shared serial
dispatch queue would freeze all relay
connections permanently, including unrelated
new connections.
- Set relay file descriptors to `O_NONBLOCK`
and handle `EAGAIN` by suspending reads and
installing a write source to drain pending data.
- Give each `BidirectionalRelay` its own serial queue
instead of sharing one from `UnixSocketRelayManager`,
eliminating cross-connection blocking.
- Resume suspended read sources before cancelling
in `stop()` — GCD does not deliver cancel handlers
on suspended dispatch sources, which caused file
descriptor and memory leaks on teardown under
backpressure.
- Guard-unwrap `buf.baseAddress` in
`drainPendingWrite`.
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