Option and Result over async streams: Choose, SequenceAsync and TraverseAsync on IAsyncEnumerable #128

Choose() and Choose(selector) take an IEnumerable<T> (OptionExtensions.cs), and every TraverseAsync awaits one operation per element of a synchronous sequence (ResultExtensions.cs). Nothing in CodoMetis.TypeKit takes an IAsyncEnumerable<T>, so a paged API, a streamed import or an EF query enumerated with AsAsyncEnumerable() has to be buffered into a list before it reaches them, or unwrapped by hand with TryGetValue in an await foreach.

Proposal

• Choose() over an IAsyncEnumerable<Option<T>> and Choose(selector) over an IAsyncEnumerable<T> with a selector returning Option<TResult>: lazy, in order, returning an IAsyncEnumerable<T>.
• SequenceAsync(cancellationToken) over an IAsyncEnumerable<Result<T, TError>> (and Result<TError>): the successes as an IReadOnlyList<T>, or the first error, reading nothing after it.
• TraverseAsync over an IAsyncEnumerable<T>, with the same overloads and the same one-at-a-time rule as the synchronous one, if the spike of the names below leaves room for it.

Before the code: .NET 10 ships System.Linq.AsyncEnumerable in the shared framework, so check that no name here binds ambiguously beside its operators, and that CMTK0003 reports a dropped Task<Result<...>> from SequenceAsync as it does for TraverseAsync.

Done when

• Tests show cancellation reaching the enumerator (WithCancellation and the token parameter), the enumerator disposed on an early exit (the first error, a break, an exception from the selector), order kept, and an uninitialized Option/Result element refused as the synchronous overloads refuse it.
• A null delegate is refused as everywhere else (NullDelegateTests covers the new members), and AsynchronousContextTests covers them.
• The core README's sequence section has a compiled sample, each new member has its claim or waiver in test/oracles/, and the changelog says so.