[Repo Assist] Add AsyncSeq.removeAt, updateAt, insertAt — 3 new Seq-mirroring combinators

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Summary

Adds three combinators that mirror the F# 6.0 Seq module additions:

val removeAt : index:int -> source:AsyncSeq<'T> -> AsyncSeq<'T>
val updateAt : index:int -> value:'T -> source:AsyncSeq<'T> -> AsyncSeq<'T>
val insertAt : index:int -> value:'T -> source:AsyncSeq<'T> -> AsyncSeq<'T>

AsyncSeq.removeAt

Returns a new sequence with the element at position index omitted:

AsyncSeq.ofSeq [1; 2; 3; 4] |> AsyncSeq.removeAt 1 // → [1; 3; 4]

AsyncSeq.updateAt

Returns a new sequence with the element at position index replaced:

AsyncSeq.ofSeq [1; 2; 3] |> AsyncSeq.updateAt 1 99 // → [1; 99; 3]

AsyncSeq.insertAt

Returns a new sequence with value inserted before position index (appends when index = length):

AsyncSeq.ofSeq [1; 2; 4] |> AsyncSeq.insertAt 2 3 // → [1; 2; 3; 4]
AsyncSeq.ofSeq [1; 2; 3] |> AsyncSeq.insertAt 3 4 // → [1; 2; 3; 4]
```

## Error Handling

All three raise `ArgumentException` for negative indices. `insertAt` additionally raises `ArgumentException` when the index exceeds the sequence length (matching `Seq.insertAt` semantics).

## Implementation Notes

- All three are implemented as `asyncSeq { for x in source do … }` generators — clean, single-pass, lazy.
- Placed immediately after `AsyncSeq.except` in `AsyncSeq.fs`, adjacent to other element-manipulation combinators.
- Signatures added to `AsyncSeq.fsi`.
- `RELEASE_NOTES.md` updated with a 4.7.0 entry.
- `version.props` bumped to 4.7.0.

## Test Status

✅ All **301 tests pass** (287 pre-existing + **14 new tests**):

```
Passed!  - Failed: 0, Passed: 301, Skipped: 0, Total: 301

New tests cover: normal operation, boundary cases (index 0, index = last, index = length), empty source, negative index, and out-of-bounds index.

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