SqlPollReader's SemaphoreSlim was never disposed and a new reader is built per
rebind. The reader is now IDisposable with a reference count (owner + one per
in-flight group): the gate is disposed only when the last group - including a
timed-out one still wedged in the provider - releases it, so nothing ever releases
into a disposed semaphore. ApplyBinding disposes the replaced reader; the final
DisposeAsync disposes the live one (Shutdown does not, since a re-init without a
rebind keeps polling through it).
Refs #744
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
16f401b9e7