Add Eventually polling assertions for eventually-consistent scenarios #3263

Integration tests against message queues, background workers, file watchers or UI state often need to wait until an assertion eventually passes. Today people hand-roll retry loops or pull in a separate library (Awaitility-style).

Proposed API:

await Eventually.Satisfy(() => queue.Count.Should().Be(3), within: 5.Seconds(), checkEvery: 100.Milliseconds());

It would repeatedly evaluate the assertion action, swallowing failures until it passes or the timeout expires, then rethrow the last failure. The internal IClock/ITimer abstraction can drive this testably. Relates to the TimeProvider idea (separate issue) for deterministic testing.