[Repo Assist] feat: add TaskSeq.unfold and TaskSeq.unfoldAsync (ref #289)

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Summary

Adds TaskSeq.unfold and TaskSeq.unfoldAsync — lazy sequence generators that produce elements from an evolving state value, without requiring the element count to be known upfront. Matches the semantics of Seq.unfold and AsyncSeq.unfold.

Part of the work to align with FSharp.Control.AsyncSeq (ref #289). Please do not close #289 as this is one PR of several.

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Implementation

Motivation

TaskSeq.init and TaskSeq.initAsync require a known count. unfold fills the gap for cases where the termination condition depends on state (e.g. parsing, pagination, Fibonacci, iterating a cursor).

Performance characteristics:

• Zero intermediate collection allocations — elements are generated on demand
• Generator called exactly once per element, plus one final None call
• Works with take/truncate to consume infinite-like sequences lazily

Changes

File	Change
TaskSeqInternal.fs	Two new taskSeq {} generators: unfold (sync) and unfoldAsync (async)
TaskSeq.fs	Expose both via TaskSeq.unfold / TaskSeq.unfoldAsync
TaskSeq.fsi	XML-doc signatures
TaskSeq.Unfold.Tests.fs	14 new tests (new file)
FSharp.Control.TaskSeq.Test.fsproj	Register test file

API

TaskSeq.unfold      : ('State -> ('T * 'State) option)         -> 'State -> TaskSeq<'T>
TaskSeq.unfoldAsync : ('State -> Task<('T * 'State) option>)   -> 'State -> TaskSeq<'T>

Example

// Fibonacci sequence (infinite), take first 10
TaskSeq.unfold (fun (a,b) -> Some (a, (b, a+b))) (1,1)
|> TaskSeq.take 10
// → 1, 1, 2, 3, 5, 8, 13, 21, 34, 55

Note on unfoldAsync signature

The generator is typed as Task<_> rather than #Task<_> (flexible type) because the taskSeq {} builder requires a type annotation to resolve the Bind overload. In practice, callers using task { return ... } expressions will satisfy this constraint directly.

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Test Status

Build: ✅ succeeded (0 warnings, 0 errors)
Tests: ✅ 14/14 passed
Fantomas: ✅ clean after formatting

Test cases:

• Empty sequence (generator returns None immediately)
• Finite sequence (0..9)
• Singleton (one element then None)
• Fibonacci via state threading
• String generation (A..Z)
• Infinite sequences truncated with take 100
• Generator call-count verification (n elements + 1 None call)
• Re-iteration restarts from initial state

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