[Repo Assist] perf+fix: optimize pairwise, fix splitAt/tryTail enumerator disposal on exception

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Summary

Two improvements in one PR: a performance optimization (Task 8) and a bug fix (Task 3).

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Task 8 — Performance: AsyncSeq.pairwise

Root cause: The previous implementation used Option boxing for the prev element (prev <- Some v on every iteration), allocating a new heap object per step.

Fix: Replace 'T option with a hasPrev: bool flag and a direct mutable prev: 'T field.

// Before — allocates Some v on every iteration
let mutable prev = None
while b.IsSome do
    match prev with
    | None -> ()
    | Some p -> yield (p, v)
    prev <- Some v   // ← heap allocation

// After — zero allocation per step
let mutable hasPrev = false
let mutable prev = Unchecked.defaultof<'T>
while b.IsSome do
    if hasPrev then yield (prev, v)
    hasPrev <- true
    prev <- v        // ← no allocation

Benefit: Eliminates one 'T option heap allocation per input element, reducing GC pressure for long sequences.

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Task 3 — Bug fix: splitAt and tryTail enumerator disposal on exception

Root cause: Both functions obtained an enumerator with let ie = source.GetEnumerator() (not use ie), intending to transfer ownership to the returned AsyncSeq. However, if the source threw an exception during the initial MoveNext() call, ie.Dispose() was never called — causing a resource leak.

Fix: Wrap the initial async body in try...with ex -> ie.Dispose(); return raise ex so the enumerator is always disposed on exception or cancellation.

Bonus: The let cur = ref b ref cell in splitAt's rest sequence was also replaced with let mutable cur = b (no allocation, same semantics).

Trade-off: The "caller discards the rest without enumerating it" case is a pre-existing limitation of this API design and is not addressed here (it would require a different return type).

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Test Status

✅ Build: 0 errors, pre-existing warnings only
✅ Tests: 425/425 passed (3 new tests added)

• AsyncSeq.pairwise with many elements produces correct pairs — validates the optimization produces correct output for a 10-element sequence
• AsyncSeq.splitAt disposes enumerator when source throws during collection — verifies disposal on exception mid-collection
• AsyncSeq.tryTail disposes enumerator when source throws on first MoveNext — verifies disposal when first async step fails

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