[Repo Assist] feat: add TaskSeq.scan and TaskSeq.scanAsync (ref #289)

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Summary

Adds TaskSeq.scan and TaskSeq.scanAsync — the "scan" analogue of fold/foldAsync, which returns the sequence of intermediate accumulator states rather than just the final value.

This is part of the ongoing work to align TaskSeq with FSharp.Control.AsyncSeq (ref #289).

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Task 10 — Forward: TaskSeq.scan / TaskSeq.scanAsync

What is scan?

Like Seq.scan, but for async task sequences:

// fold returns only the final state
let! total = TaskSeq.ofList [1;2;3;4;5] |> TaskSeq.fold (+) 0
// total = 15

// scan returns all intermediate states (N+1 elements for N-element input)
let! running = TaskSeq.ofList [1;2;3;4;5] |> TaskSeq.scan (+) 0 |> TaskSeq.toListAsync
// running = [0; 1; 3; 6; 10; 15]

Changes

File	Change
TaskSeqInternal.fs	scan function using FolderAction DU (same pattern as fold)
TaskSeq.fsi	scan and scanAsync signatures with XML doc
TaskSeq.fs	Static member dispatch for scan/scanAsync
TaskSeq.Scan.Tests.fs	54 new tests covering empty, single-element, multi-element, string accumulation, immutable variants, and side-effect variants
FSharp.Control.TaskSeq.Test.fsproj	Added TaskSeq.Scan.Tests.fs to compile order

Implementation

The implementation is a natural generalisation of fold: it yields the initial state, then for each element it updates the state and yields it again.

let scan folder initial (source: TaskSeq<_>) =
    checkNonNull (nameof source) source
    match folder with
    | FolderAction folder -> taskSeq {
        let mutable state = initial
        yield state
        for item in source do
            state <- folder state item
            yield state
      }
    | AsyncFolderAction folder -> taskSeq {
        let mutable state = initial
        yield state
        for item in source do
            let! newState = folder state item
            state <- newState
            yield state
      }

Test Status

✅ Build succeeded (Release)
✅ 3598 tests passed, 2 skipped (infrastructure skips, pre-existing)
✅ Fantomas formatting check passes
✅ 54 new scan/scanAsync tests all pass

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Commit pushed: ad62805

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