[Repo Assist] Add AsyncSeq.insertManyAt, removeManyAt, splitInto

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Summary

Adds three API functions for parity with F# 6 collection modules (Seq, List, Array):

Function	Description	Mirrors
AsyncSeq.insertManyAt	Inserts multiple values before the element at the specified index	Seq.insertManyAt / List.insertManyAt
AsyncSeq.removeManyAt	Removes count elements starting at the specified index	Seq.removeManyAt / List.removeManyAt
AsyncSeq.splitInto	Splits into at most count chunks of as-equal-as-possible size	Seq.splitInto / Array.splitInto

Motivation

The existing API already has insertAt, removeAt, and updateAt (added in 4.11/4.15). The natural companions — insertManyAt, removeManyAt — were missing. Similarly, chunkBySize exists but splitInto (which divides into N roughly-equal chunks) had no AsyncSeq equivalent.

Implementation Details

• insertManyAt: lazy asyncSeq that yields the values sequence at the insertion point; yield! values handles any seq<'T> efficiently.
• removeManyAt: lazy asyncSeq that skips elements in the [index, index+count) range; validates bounds after enumeration (consistent with removeAt/insertAt style).
• splitInto: materialises to an array (same approach as sort/rev/partition), then uses the standard F# formula: first total mod count chunks get (total / count) + 1 elements, the rest get total / count. Uses Operators.min to avoid shadowing by the module-level AsyncSeq.min.
• .fsi signature file updated with XML doc comments.
• RELEASE_NOTES.md updated under the 4.16.0 entry.

Test Status

✅ Build: 0 errors, pre-existing warnings only
✅ Tests: 442/442 passed (11 new tests added covering normal, edge, and error cases for all three functions)

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