Add true trailing-edge debounce scheduler

[dombrovsky]

Summary

Add a true trailing-edge debounce policy that waits for a quiet interval and eventually executes the latest submitted operation. This must be distinct from the current leading-edge rejection policy, which is being named WithThrottle.

Design questions

• Is the quiet interval measured from the most recent Enqueue call, including time spent waiting behind existing work?
• How do superseded operation tasks complete: canceled, faulted with a dedicated exception, or linked to the latest result?
• Does a newer submission cancel only pending work, or also request cancellation of an operation that has already started?
• Does the debounce timer occupy the underlying sequential lane while it waits?
• How are pending debounce operations owned and canceled when the underlying flow is disposed?
• How should generic result types and caller cancellation compose across replacements?
• How should exact interval-boundary races be resolved?

Acceptance criteria

• The API and XML documentation define trailing-edge behavior precisely and distinguish it from throttling and CreateCancelPrevious.
• Rapid submissions invoke only the final eligible delegate after the configured quiet interval.
• Superseded returned tasks have deterministic, documented completion behavior.
• Caller cancellation and flow disposal cannot leave pending tasks incomplete.
• The implementation is thread-safe and uses TimeProvider for deterministic timing.
• Tests cover TaskFlow and DedicatedThreadTaskFlow, exact-boundary races, cancellation, disposal, delegate failures, and supported target frameworks.
• Examples avoid ambiguous Task<T>/ValueTask<T> async lambdas.