[Repo Assist] feat: add TaskSeq.allPairs (69 tests)

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Implements TaskSeq.allPairs, which returns a task sequence of all pairings of elements from two input task sequences — matching the behaviour of Seq.allPairs.

API

TaskSeq.allPairs : source1: TaskSeq<'T1> -> source2: TaskSeq<'T2> -> TaskSeq<'T1 * 'T2>
```

`source2` is fully materialised (into a `ResizeArray`) before the result sequence is iterated. `source1` is consumed lazily. Output is in row-major order: for each element of `source1`, all elements of `source2` appear in sequence.

## Files changed

- `TaskSeqInternal.fs` — `allPairs` implementation using `toResizeArrayAsync` + nested `for` loops inside `taskSeq`
- `TaskSeq.fs` — public `static member` wrapper
- `TaskSeq.fsi` — XML-documented signature (after `zip3`)
- `TaskSeq.AllPairs.Tests.fs` — 69 new tests (new file)
- `FSharp.Control.TaskSeq.Test.fsproj` — includes new test file
- `README.md` — marks `allPairs` as ✅ implemented
- `release-notes.txt` — updated

## Test Status

Build and full test suite verified:

```
Build succeeded.
    0 Warning(s)
    0 Error(s)

Passed! - Failed: 0, Passed: 4599, Skipped: 2, Total: 4601

Formatting verified with dotnet fantomas . --check — no issues.

Design notes

• source2 must be pre-materialised because it is iterated once per element of source1. The XML doc makes this explicit.
• The implementation mirrors how Seq.allPairs works: nested iteration with the outer loop over source1.
• No new DU variants or abstractions needed — the implementation is a single taskSeq expression.
• source1 is never materialised, so allPairs works correctly on infinite sequences as source1.

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