[Repo Assist] feat/test: add TaskSeq.pairwise; expand distinctUntilChanged tests

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Summary

Two improvements: a new TaskSeq.pairwise function (addressing #289), and substantially expanded tests for TaskSeq.distinctUntilChanged.

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Task 10 — Forward: TaskSeq.pairwise (ref #289)

Implements pairwise to align with FSharp.Control.AsyncSeq.pairwise, as requested in #289. This is the first of what should be multiple incremental PRs toward full AsyncSeq alignment.

Semantics: Returns a task sequence of consecutive overlapping pairs (a₀,a₁), (a₁,a₂), …. Returns empty if the source has fewer than two elements. Mirrors Seq.pairwise.

Implementation (TaskSeqInternal.fs): uses the same ValueNone/ValueSome state pattern as distinctUntilChanged — no heap allocation beyond the single ValueOption slot:

let pairwise (source: TaskSeq<_>) =
    checkNonNull (nameof source) source
    taskSeq {
        let mutable maybePrevious = ValueNone
        for current in source do
            match maybePrevious with
            | ValueNone -> maybePrevious <- ValueSome current
            | ValueSome previous ->
                yield previous, current
                maybePrevious <- ValueSome current
    }

Changes: TaskSeqInternal.fs (impl), TaskSeq.fs (static member), TaskSeq.fsi (signature + XML doc).

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Task 9 — Testing: expand distinctUntilChanged + new pairwise tests

TaskSeq.DistinctUntilChanged.Tests.fs — from 39 → ~110 lines

The file previously had only 2 tests (one null, one functionality). Added:

Functionality module:

• Singleton input → singleton output
• All identical elements → single element output
• All distinct elements → all returned
• Alternating pairs [A;A;B;B;A;A] → [A;B;A]
• TestImmTaskSeq variant test (1..10 all unique, all preserved)

SideEffects module:

• Verifies every source element is consumed exactly once
• Verifies duplicate elements are skipped without extra evaluation (counter stays at 5 for [1;1;2;2;3])
• TestSideEffectTaskSeq variant test

TaskSeq.Pairwise.Tests.fs — new file

Full coverage following the established module structure:

• EmptySeq: null arg, empty variants, singleton → empty
• Immutable: two-element, 1..5 sequence, length formula, shared elements ((A,B),(B,C) — right of pair i = left of pair i+1), TestImmTaskSeq variants
• SideEffects: exact consumption count, TestSideEffectTaskSeq variants

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Test Status

• Build: ✅ succeeded (Release, .NET 9 / netstandard2.1)
• Tests: ✅ 3597 passed, 2 skipped, 0 failed (up from 3564 prior to this branch — 33 new tests)
• Formatting: ✅ Fantomas check passed

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[dsyme]

/repo-assist Update the release notes please

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Commit pushed: b96a5ab

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Commit pushed: 9aa0b3d

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