Pool bounded async-enumerable workers

[thomhurst]

Summary

• replace bounded async-stream semaphore fan-out with a bounded Channel<T> and P persistent workers
• cap ordered result buffering at P with Queue<Task<T>>, preserving input-order yields without List.RemoveAt(0) shifts
• collect no-selector async streams directly instead of scheduling no-op tasks
• remove redundant Task.Run(async () => await ...) wrappers from unbounded paths
• add explicit ordering and source-backpressure regression tests

Impact

Bounded async-stream coordination now scales with configured concurrency instead of item count. Producers cannot run arbitrarily ahead, synchronous delegate prefixes run on worker threads, and result processors retain ordered streaming.

Validation

• bounded processor tests: 17 passed
• async-enumerable extension tests: 11 passed
• searches confirm no bounded semaphore fan-out, RemoveAt(0), or redundant async Task.Run wrapper remains
• dotnet test: 1,617 tests passed across net8.0, net9.0, and net10.0

Benchmark dependency

PR #353 owns the new benchmark project and is still open. The async-stream comparison should be added there after that project lands; this PR avoids duplicating the entire benchmark scaffold and creating a guaranteed conflict.

Closes #337

[greptile-apps]

Greptile Summary

This PR replaces bounded async-enumerable fan-out with a persistent worker pool. The main changes are:

• Uses bounded channels to limit queued work and source read-ahead.
• Preserves input order with a bounded queue of result tasks.
• Simplifies selector-free collection and unbounded task scheduling.
• Adds ordering, backpressure, and concurrent-failure tests.

Confidence Score: 5/5

This looks safe to merge.

• Concurrent worker failures now surface an original selector exception directly.
• The new test covers overlapping failures in the bounded action path.
• No blocking issue related to the earlier exception-handling failure remains.

Important Files Changed

Filename	Overview
EnumerableAsyncProcessor/AsyncEnumerableWorkerPool.cs	Adds bounded channel workers, ordered result buffering, cancellation cleanup, and direct propagation of the first captured action failure.
EnumerableAsyncProcessor.UnitTests/AsyncEnumerableProcessorTests.cs	Adds tests for ordered results, source backpressure, and overlapping worker failures.
EnumerableAsyncProcessor/Extensions/AsyncEnumerableExtensions.cs	Routes bounded result processing through the worker pool and simplifies collection and task scheduling.
EnumerableAsyncProcessor/RunnableProcessors/AsyncEnumerable/AsyncEnumerableParallelProcessor.cs	Replaces bounded semaphore fan-out with the shared worker pool.
EnumerableAsyncProcessor/RunnableProcessors/AsyncEnumerable/ResultProcessors/ResultAsyncEnumerableParallelProcessor.cs	Uses bounded worker-pool streaming while retaining the existing unbounded path.

_{Reviews (6): Last reviewed commit: "fix: preserve async stream exception typ..." | Re-trigger Greptile}