[Repo Assist] feat: add TaskSeq.map3 and TaskSeq.map3Async (81 tests)

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Implements TaskSeq.map3 and TaskSeq.map3Async, which apply a mapping function to the corresponding elements of three task sequences in parallel, stopping when the shortest sequence is exhausted — matching the behaviour of Seq.map3.

API

TaskSeq.map3      : mapping: ('T1 -> 'T2 -> 'T3 -> 'U)         -> source1: TaskSeq<'T1> -> source2: TaskSeq<'T2> -> source3: TaskSeq<'T3> -> TaskSeq<'U>
TaskSeq.map3Async : mapping: ('T1 -> 'T2 -> 'T3 -> #Task<'U>) -> source1: TaskSeq<'T1> -> source2: TaskSeq<'T2> -> source3: TaskSeq<'T3> -> TaskSeq<'U>
```

## Files changed

- `TaskSeqInternal.fs` — `map3` / `map3Async` implementations, modelled on the existing `zip3`
- `TaskSeq.fs` — public `static member` wrappers
- `TaskSeq.fsi` — XML-documented signatures (after `mapiAsync`)
- `TaskSeq.Map3.Tests.fs` — 81 new tests (new file)
- `FSharp.Control.TaskSeq.Test.fsproj` — includes new test file
- `README.md` — marks `map3` / `map3Async` 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: 4611, Skipped: 2, Total: 4613

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

Trade-offs / Notes

• Implementation is a direct parallel of zip3: opens three enumerators, advances them simultaneously on each step, stops at the first exhausted sequence.
• No new DU cases needed; map3 and map3Async are top-level functions (unlike map/mapi which share the Action DU), because their arity and type-safety requirements are different.
• This PR is independent of the pending map2 PR ([Repo Assist] feat: add TaskSeq.map2, map2Async, iter2, iter2Async (124 tests) #322) — no merge dependency.

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