Fix Fable compatibility: guard getIterator obsolete attribute and complete #if FABLE_COMPILER cleanup

[Copilot]

Addresses remaining Fable build errors from the AsyncSeq v4 IAsyncEnumerable<'T> migration and cleans up [<Obsolete>] attribute handling for Fable vs .NET builds.

Fable build errors (fixed in previous sub-PR, now merged)

• ValueTask<T> unavailable in Fable — guarded AsyncSeqImpl<'T>'s IAsyncEnumerator implementation with #if !FABLE_COMPILER
• IAsyncEnumerable<_>.GetEnumerator not found — for Fable, AsyncSeq<'T> is now defined as an interface with a public GetEnumerator() returning IAsyncSeqEnumerator<'T>, avoiding BCL types unavailable in Fable

[<Obsolete>] attribute cleanup

getIterator in AsyncSeq.fsi was unconditionally marked [<System.Obsolete("Use .GetEnumerator directly")>], but the message only makes sense for Fable (where GetEnumerator() is a public interface method on AsyncSeq<'T>). In .NET builds, GetEnumerator() is an internal extension method not exposed in .fsi, making the guidance incorrect.

Guard the attribute:

#if FABLE_COMPILER
[<System.Obsolete("Use .GetEnumerator directly") >]
#endif
val getIterator : source:AsyncSeq<'T> -> (unit -> Async<'T option>)

ofAsyncEnum/toAsyncEnum are already marked [<Obsolete>] and correctly excluded from Fable via #if !FABLE_COMPILER (they reference IAsyncEnumerable<'T> which doesn't exist in Fable).

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