[Repo Assist] Add AsyncSeq.sumBy, sumByAsync, average, averageBy, averageByAsync

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Summary

Adds five new aggregation combinators that mirror the corresponding Seq module functions, complementing the existing AsyncSeq.sum:

AsyncSeq.sumBy        : projection:('T -> ^U) -> source:AsyncSeq<'T>            -> Async< ^U>
AsyncSeq.sumByAsync   : projection:('T -> Async< ^U>) -> source:AsyncSeq<'T>    -> Async< ^U>
AsyncSeq.average      : source:AsyncSeq< ^T>                                    -> Async< ^T>
AsyncSeq.averageBy    : projection:('T -> ^U) -> source:AsyncSeq<'T>            -> Async< ^U>
AsyncSeq.averageByAsync : projection:('T -> Async< ^U>) -> source:AsyncSeq<'T> -> Async< ^U>

The average* variants raise InvalidArgumentException when the sequence is empty (same as Seq.average).

Motivation

AsyncSeq.sum already exists, and sumBy/average/averageBy are the natural companions. Users frequently need to compute weighted sums and averages over async streams (e.g. aggregating metrics, sensor readings). Having these avoids users reaching for toArrayAsync + Array.average which buffers the entire sequence.

Implementation

All functions are inline with the appropriate SRTP constraints (same pattern as sum):

• sumBy / sumByAsync — fold with GenericZero accumulator, applying projection first
• average / averageBy / averageByAsync — fold tracking (sum, count), then call LanguagePrimitives.DivideByInt

Test Status

✅ Build: succeeded (no errors, 2 pre-existing warnings for groupByAsync)
✅ Tests: all 200 tests pass (194 existing + 6 new)

New tests cover: basic correctness against List.sumBy/List.averageBy, async projections, and empty-sequence exception for average.

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[github-actions]

Pull request created: #245