Core.Abstractions-001: PollGroupEngine compares array values with structural
equality so a driver returning a fresh T[] each poll no longer fires spuriously.
Core.Abstractions-002: PollOnceAsync guards reader result cardinality and
throws a descriptive InvalidOperationException on mismatch instead of a
swallowed ArgumentOutOfRangeException that stalled the subscription.
Core.Abstractions-003: the poll loop Task is tracked; Unsubscribe/DisposeAsync
await loop completion before disposing the CTS.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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