One SemaphoreSlim serialised reads, writes, subscribes, acks and the whole recursive
discovery browse, so a stalled write (up to the operation timeout) blocked every
read and subscription create, and a discovery browse would block the data plane for
its full duration. Reads/acks keep the read gate; writes and DiscoverAsync are
ungated (the SDK session is safe for concurrent service calls, the host bounds write
concurrency); subscription management takes its own gate. Dispose drains both
(DisposeAsync's drain no longer over-releases on a timed-out wait).
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
66d100a9e9