[Repo Assist] Perf: optimise mapiAsync with direct enumerator

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Summary

Replaces the asyncSeq-builder + collect implementation of mapiAsync with a new OptimizedMapiAsyncEnumerator type that iterates the source directly and maintains the index in a mutable field.

Before (asyncSeq builder path):

let mapiAsync f (source : AsyncSeq<'T>) : AsyncSeq<'TResult> = asyncSeq {
    let i = ref 0L
    for itm in source do    // → calls collect / asyncSeq.For overhead
      let! v = f i.Value itm
      i := i.Value + 1L
      yield v }

After (direct enumerator, same pattern as mapAsync):

type private OptimizedMapiAsyncEnumerator<'T, 'TResult>(...) =
    let mutable index = 0L
    interface IAsyncSeqEnumerator<'TResult> with
      member _.MoveNext() = async {
        let! moveResult = source.MoveNext()
        match moveResult with
        | None -> return None
        | Some value ->
            let i = index
            index <- index + 1L
            let! mapped = f i value
            return Some mapped }

Why

mapiAsync is a core transformation used by mapi and indexed. The previous implementation went through asyncSeq.For → collect, which allocates a generator per element. The new implementation is a thin enumerator wrapper with no per-element allocation overhead, matching the approach already used by mapAsync, filterAsync, and chooseAsync.

Benchmarks added

New AsyncSeqMapiBenchmarks class in the benchmarks project measures mapAsync (baseline), mapiAsync, and mapi at 1K and 10K elements.

Test Status

✅ Build succeeded (netstandard2.0, warnings only — pre-existing).
✅ All 321/321 tests passed.

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