[Repo Assist] Add AsyncSeq.windowed for sliding-window combinators

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Summary

Adds AsyncSeq.windowed : windowSize:int -> source:AsyncSeq<'T> -> AsyncSeq<'T []> — a sliding-window combinator analogous to Seq.windowed.

Each call yields overlapping windows of windowSize elements as immutable 'T[] snapshots:

asyncSeq { yield 1; yield 2; yield 3; yield 4; yield 5 }
|> AsyncSeq.windowed 3
|> AsyncSeq.toListSynchronously
// [[|1;2;3|]; [|2;3;4|]; [|3;4;5|]]

Notable properties (matching Seq.windowed):

• Yields the first window once windowSize elements have been consumed, then slides one element at a time.
• Returns an empty sequence if the source has fewer than windowSize elements.
• Raises ArgumentException if windowSize < 1.
• AsyncSeq.pairwise is a special case equivalent to AsyncSeq.windowed 2 (but returning tuples instead of arrays).

Implementation

Uses System.Collections.Generic.Queue<'T> with an initial capacity of windowSize so each element costs one O(1) enqueue + one O(1) dequeue (when the window is full).

Motivation

Seq.windowed is a standard F# core library function. Its absence from AsyncSeq is a gap users commonly encounter when porting sequential code to async streaming — e.g. computing moving averages, detecting consecutive patterns, or correlating adjacent frames. This PR fills that gap with minimal surface area.

Test Status

✅ All 201 tests pass (dotnet test). 7 new tests cover:

1. Empty source → empty result
2. Source shorter than window → empty result
3. Source exactly equal to window size → single window
4. Sliding windows (5 elements, window=3 → 3 windows)
5. Window size 1 → each element as a singleton array
6. Window size 2 → equivalent to pairwise as arrays
7. Window size 0 → ArgumentException

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✅ Pull request created: #241