Fix head-of-line blocking and resource leak in BidirectionalRelay

[mlsad3]

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.

Closes #712

Changes

• 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

Test plan

Added BidirectionalRelayTests with 5 tests:

• Basic bidirectional relay
• Cross-connection head-of-line blocking isolation
• Backpressure recovery (EAGAIN → write source drain → resume)
• EOF handling
• Stop while backpressured (verifies cancel handlers fire and waitForCompletion() resolves)

Tested under sustained concurrent load that previously caused permanent hangs; verified normal operations work correctly after sustained load.