[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` — 81new 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 andfull 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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