[Repo Assist] perf: replace ValueOption tracking with while! in pairwise and distinctUntilChanged family

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Summary

pairwise, distinctUntilChanged, distinctUntilChangedWith, and distinctUntilChangedWithAsync previously used a ValueOption mutable and a for current in source do loop to track the "previous" element:

// before
let mutable maybePrevious = ValueNone

for current in source do
    match maybePrevious with
    | ValueNone -> maybePrevious <- ValueSome current
    | ValueSome previous ->
        yield previous, current
        maybePrevious <- ValueSome current

This PR replaces each with an explicit enumerator + while!, consistent with the pattern already used in except, exceptOfSeq, skipWhile, and similar functions:

// after
use e = source.GetAsyncEnumerator CancellationToken.None
let! hasFirst = e.MoveNextAsync()

if hasFirst then
    let mutable previous = e.Current

    while! e.MoveNextAsync() do
        let current = e.Current
        yield previous, current
        previous <- current

Why this is better

• Eliminates per-element struct match: Every element previously required a ValueNone / ValueSome branch. A direct mutable avoids this entirely.
• Structural consistency: Uses the same while! + explicit enumerator idiom found throughout the rest of the optimised functions in TaskSeqInternal.fs.
• Cleaner code: The first-element sentinel logic is made explicit via a single let! hasFirst = e.MoveNextAsync() check rather than inlined as a DU branch per element.

What changed

• src/FSharp.Control.TaskSeq/TaskSeqInternal.fs: refactored pairwise, distinctUntilChanged, distinctUntilChangedWith, distinctUntilChangedWithAsync
• release-notes.txt: entry under Unreleased

Test Status

• ✅ dotnet build src/FSharp.Control.TaskSeq.sln -c Release — clean, 0 warnings, 0 errors
• ✅ dotnet test (Release) — 5251 passed, 0 failed, 2 skipped
• ✅ Targeted: Pairwise + DistinctUntilChanged suites — 94 passed, 0 failed
• ✅ dotnet fantomas src/FSharp.Control.TaskSeq/TaskSeqInternal.fs --check — no formatting changes

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