[Repo Assist] feat: add TaskSeq.map2, map2Async, iter2, iter2Async (124 tests)

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🤖 This is a draft PR from Repo Assist, an automated AI assistant.

Implements the first batch of '2'-functions from the README roadmap (exists2 / map2 / fold2 / iter2 and related), covering the most commonly used pair:

TaskSeq.iter2 :('T -> 'U -> unit)-> TaskSeq<'T>-> TaskSeq<'U>-> Task(unit)
TaskSeq.iter2Async :('T -> 'U -> #Task(unit))-> TaskSeq<'T>-> TaskSeq<'U>-> Task(unit)
TaskSeq.map2 :('T -> 'U -> 'V)-> TaskSeq<'T>-> TaskSeq<'U>-> TaskSeq<'V>
TaskSeq.map2Async :('T -> 'U -> #Task<'V>)-> TaskSeq<'T>-> TaskSeq<'U>-> TaskSeq<'V>

Semantics (matching zip): when the two input sequences have different lengths, iteration stops at the shorter sequence — excess elements in the longer sequence are silently ignored.

Files changed

• src/FSharp.Control.TaskSeq/TaskSeqInternal.fs — iter2, iter2Async, map2, map2Async implementations; placed adjacent to iter and map respectively
• src/FSharp.Control.TaskSeq/TaskSeq.fs — wrapper static members
• src/FSharp.Control.TaskSeq/TaskSeq.fsi — public signature with XML doc; placed after iteriAsync and mapiAsync
• src/FSharp.Control.TaskSeq.Test/TaskSeq.Map2Iter2.Tests.fs — 124 new tests: null checks, empty sequences, truncation, type mixing, side-effect sequences, immutable variants
• 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 iter2/iter2Async and map2/map2Async as ✅ in the table; update checklist

Implementation notes

The implementations follow the exact same pattern as zip/zip3 (manual enumerator loop, let mutable go = true, stop when either sequence is exhausted). This keeps them consistent with the existing codebase rather than layering on top of zip.

map2/map2Async return a lazy taskSeq { ... } (no allocation until enumerated). iter2/iter2Async return an eager task { ... } that exhausts both sequences immediately, matching iter behaviour.

AI Disclosure

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

Test Status

✅ Build: succeeded (Release configuration, netstandard2.1)
✅ Tests: 4654 passed, 2 skipped (existing skips), 0 failed — full suite
✅ New tests: 124 tests in TaskSeq.Map2Iter2.Tests.fs
✅ Formatting:dotnet fantomas . --check passes

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