[Repo Assist] Add AsyncSeq.splitAt — split at index, returning first N as array and remainder as AsyncSeq

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Summary

Adds AsyncSeq.splitAt, mirroring Seq.splitAt:

val splitAt : count:int -> source:AsyncSeq<'T> -> Async<'T array * AsyncSeq<'T>>

Returns an Async computation that yields:

• the first count elements as a 'T array
• the remaining elements as a new AsyncSeq<'T>

The source is enumerated once — the underlying enumerator is shared across the split boundary. The returned remainder AsyncSeq lazily produces remaining elements and disposes the enumerator when fully consumed.

Usage Example

let source = asyncSeq { yield 1; yield 2; yield 3; yield 4; yield 5 }
let first, rest = AsyncSeq.splitAt 3 source |> Async.RunSynchronously
// first: [| 1; 2; 3 |]
// rest:  asyncSeq yielding 4, 5

let restArr = rest |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
// [| 4; 5 |]
```

Edge cases handled:
- `count = 0` → empty first array, full remainder
- `count >= length` → all elements in first array, empty remainder
- empty source → empty first array, empty remainder
- negative `count` → raises `ArgumentException`

## Implementation Notes

- Placed after `tail` in `AsyncSeq.fs`, adjacent to `skip`/`take`/`tail`
- Manual enumerator management (no `use`) allows the enumerator to be handed off to the remainder `AsyncSeq`; `try/finally` in the `asyncSeq` block ensures proper disposal
- Signature added to `AsyncSeq.fsi`
- `RELEASE_NOTES.md` updated with a 4.7.0 entry

## Test Status

✅ All **293 tests pass** (287 pre-existing + **6 new tests** for `splitAt`):

```
Passed!  - Failed: 0, Passed: 293, Skipped: 0, Total: 293

New tests cover: normal split, count=0, count≥length, empty source, exact split, and negative count.

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