[Repo Assist] Add AsyncSeq.sortByAsync and sortByDescendingAsync

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Summary

Adds two new async sort functions that fill a small gap in the AsyncSeq API and are consistent with the existing *Async pattern established by minByAsync, maxByAsync, countByAsync, and sumByAsync.

Function	Description	Pattern
sortByAsync	Sorts by an asynchronous key projection; returns Async(array<'T)>	minByAsync / maxByAsync
sortByDescendingAsync	Same as above but in descending key order	sortByDescending sync twin

Motivation

AsyncSeq already has sortBy (sync projection) and sortByDescending. The common *Async twins for projection-based operations (minByAsync, maxByAsync, countByAsync, sumByAsync) are all present. sortByAsync / sortByDescendingAsync were the only obvious gap.

Use case example: ordering query results by an async-computed property (e.g. a cached value fetched from a remote store):

let sorted =
    entities
    |> AsyncSeq.sortByAsync (fun e -> fetchScore e.Id)   // Async(int)
    |> Async.RunSynchronously

Implementation

Both functions use mapAsync to pair each element with its projection key (computing the key exactly once), then toArrayAsync to materialise, then a standard Array.sort* call:

let sortByAsync projection source = async {
    let! pairs =
        source
        |> mapAsync (fun x -> async { let! k = projection x in return (k, x) })
        |> toArrayAsync
    return pairs |> Array.sortBy fst |> Array.map snd }

Changes

• src/FSharp.Control.AsyncSeq/AsyncSeq.fs — 2 new function implementations
• src/FSharp.Control.AsyncSeq/AsyncSeq.fsi — 2 new public signatures with XML-doc
• tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs — 5 new tests
• RELEASE_NOTES.md — 4.10.1 entry

New Tests

Test	What it checks
sortByAsync sorts ascending by async projection	Correct ascending order by string length
sortByAsync returns empty array for empty sequence	Edge case — empty input
sortByAsync computes projection exactly once per element	No redundant projection calls
sortByDescendingAsync sorts descending by async projection	Correct descending order
sortByDescendingAsync returns empty array for empty sequence	Edge case — empty input

Test Status

✅ Build succeeded (0 errors, pre-existing warnings only — NU1605, FS9999 for groupByAsync).

✅ All 326 tests pass — 5 new, 321 pre-existing.

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