[Repo Assist] Add AsyncSeq.transpose — mirrors Seq.transpose

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Summary

Adds AsyncSeq.transpose, which transposes a sequence of sequences so that the i-th element of the result is an array of the i-th elements of all inner sequences. This mirrors Seq.transpose from the F# standard library.

Behaviour

• The entire source is buffered before any elements are yielded.
• All inner sequences must have the same length; mismatched lengths raise ArgumentException.
• An empty source yields an empty sequence.
• Not available under Fable (consistent with rev, sort, etc.).

Example

let rows = asyncSeq { yield [1; 2; 3]; yield [4; 5; 6] }
let cols = AsyncSeq.transpose rows |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
// cols = [| [|1;4|]; [|2;5|]; [|3;6|] |]

Changes

• src/FSharp.Control.AsyncSeq/AsyncSeq.fs — implementation of transpose
• src/FSharp.Control.AsyncSeq/AsyncSeq.fsi — signature declaration
• tests/.../AsyncSeqTests.fs — 6 new tests (basic transpose, single row/column, empty, square matrix, length mismatch)
• RELEASE_NOTES.md — entry for 4.9.0

Test Status

✅ All 323/323 tests pass (dotnet test tests/FSharp.Control.AsyncSeq.Tests/...)

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