[Repo Assist] feat: add TaskSeq.distinctUntilChangedWith and distinctUntilChangedWithAsync

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Summary

Adds two new functions that extend TaskSeq.distinctUntilChanged with a caller-supplied equality predicate:

TaskSeq.distinctUntilChangedWith      : ('T -> 'T -> bool)        -> TaskSeq<'T> -> TaskSeq<'T>
TaskSeq.distinctUntilChangedWithAsync : ('T -> 'T -> #Task(bool)) -> TaskSeq<'T> -> TaskSeq<'T>

Both yield elements from the source, skipping any element that compares equal to its predecessor according to the supplied comparer — exactly like distinctUntilChanged but with a custom equality function instead of structural (=).

Motivation

This is one of the "Group 1" additions identified in the AsyncSeq/TaskSeq design-parity gap analysis (issue #345). FSharp.Control.AsyncSeq has distinctUntilChangedWith / distinctUntilChangedWithAsync and these are a natural extension to the existing distinctUntilChanged.

Use cases:

• Case-insensitive deduplication: distinctUntilChangedWith (fun a b -> String.Equals(a, b, StringComparison.OrdinalIgnoreCase))
• Fuzzy numeric comparison: distinctUntilChangedWith (fun a b -> abs (a - b) <= tolerance)
• Projected equality: compare on a field without materialising a key sequence

Changes

File	Change
TaskSeqInternal.fs	Added distinctUntilChangedWith and distinctUntilChangedWithAsync
TaskSeq.fs	Added two static dispatch members
TaskSeq.fsi	Added XML-documented signatures
TaskSeq.DistinctUntilChanged.Tests.fs	40 new tests (63 total in the file)
release-notes.txt	Updated under 1.0.0

Trade-offs

• No 'T: equality constraint on the With variants — the caller supplies the equality function, so no structural equality constraint is needed. This is intentional and consistent with how distinctBy works.
• The async variant's comparer type is 'T -> 'T -> #Task(bool), following the same pattern as distinctByAsync and other async overloads in the library.

Test Status

✅ Build: dotnet build src/FSharp.Control.TaskSeq.sln -c Release — succeeded, 0 warnings, 0 errors
✅ Format: dotnet fantomas . --check — all files correctly formatted
✅ Tests: dotnet test ... -c Release — 4,741 passed, 2 skipped (infrastructure), 0 failed

Relates to #345.

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