[Repo Assist] feat: add TaskSeq.iteri2, iteri2Async, mapi2, mapi2Async

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Implements the indexed-pair variants from the README roadmap:

TaskSeq.iteri2 :(int -> 'T -> 'U -> unit)-> TaskSeq<'T>-> TaskSeq<'U>-> Task(unit)
TaskSeq.iteri2Async :(int -> 'T -> 'U -> #Task(unit))-> TaskSeq<'T>-> TaskSeq<'U>-> Task(unit)
TaskSeq.mapi2 :(int -> 'T -> 'U -> 'V)-> TaskSeq<'T>-> TaskSeq<'U>-> TaskSeq<'V>
TaskSeq.mapi2Async :(int -> 'T -> 'U -> #Task<'V>)-> TaskSeq<'T>-> TaskSeq<'U>-> TaskSeq<'V>

Semantics: iteration stops when either sequence is exhausted — excess elements in the longer sequence are silently ignored. This matches zip, map2, and iter2.

Files changed

• src/FSharp.Control.TaskSeq/TaskSeqInternal.fs — iteri2, iteri2Async, mapi2, mapi2Async implementations; placed adjacent to the existing iter/map functions
• src/FSharp.Control.TaskSeq/TaskSeq.fs — wrapper static members, after iteriAsync and mapiAsync
• src/FSharp.Control.TaskSeq/TaskSeq.fsi — public signatures with XML doc; placed after iteriAsync and mapiAsync respectively
• src/FSharp.Control.TaskSeq.Test/TaskSeq.Iteri2Mapi2.Tests.fs — new test file with null checks, empty sequences, truncation tests, index verification, type mixing, and side-effect tests
• src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj — new test file reference
• release-notes.txt — entry under v0.6.0
• README.md — mark iteri2/iteri2Async and mapi2/mapi2Async as ✅

Implementation notes

The implementations follow the manual enumerator-loop pattern used by zip/zip3 (and the pending map2/iter2 in PR #322). This is independent of PR #322 — these functions can be merged in any order.

For mapi2Async, an explicit upcast :> Task<_> is needed on the mapper call to help the F# compiler resolve the Bind overload (same pattern needed when using #Task<_> inside a taskSeq block without going through the Action DU).

AI Disclosure

This PR was created by Repo Assist, an automated AI assistant.

Test Status

✅ Build: succeeded (Release configuration, netstandard2.1)
✅ Tests: 4651 passed, 2 skipped (existing skips), 0 failed — full suite
✅ Formatting:dotnet fantomas . --check passes

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