A caller that already holds a live runtime instance (the QUIC realtime
provider, handing a reference to each newly accepted peer) needs one more
reference without crossing an async boundary. `open()` itself has no I/O
to await, but being `async` still forces every caller through a microtask
hop; `retain()` just increments the shared refcount and returns `this`,
synchronously, so the caller stays inside the same turn.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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