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Implements the next batch of '2'-functions from the README roadmap, following the map2/iter2 batch (PR #322):

TaskSeq.exists2     : ('T -> 'U -> bool)            -> TaskSeq<'T> -> TaskSeq<'U> -> Task(bool)
TaskSeq.forall2     : ('T -> 'U -> bool)            -> TaskSeq<'T> -> TaskSeq<'U> -> Task(bool)
TaskSeq.forall2Async: ('T -> 'U -> #Task(bool))     -> TaskSeq<'T> -> TaskSeq<'U> -> Task(bool)
TaskSeq.fold2       : ('State -> 'T -> 'U -> 'State) -> 'State -> TaskSeq<'T> -> TaskSeq<'U> -> Task<'State>
TaskSeq.fold2Async  : ('State -> 'T -> 'U -> #Task<'State>) -> 'State -> TaskSeq<'T> -> TaskSeq<'U> -> Task<'State>

Semantics: when the two input sequences have different lengths, iteration stops at the shorter sequence — extra elements of the longer sequence are silently ignored. This is consistent with zip, map2, and iter2.

Note: exists2 has no async variant (matching the F# standard library pattern for Seq.exists2).

Files changed

• src/FSharp.Control.TaskSeq/TaskSeqInternal.fs — fold2, fold2Async, forall2, forall2Async, exists2 implementations; placed adjacent to fold, forall, exists respectively
• src/FSharp.Control.TaskSeq/TaskSeq.fs — wrapper static members
• src/FSharp.Control.TaskSeq/TaskSeq.fsi — public signatures with XML doc; placed after their single-source counterparts
• src/FSharp.Control.TaskSeq.Test/TaskSeq.Exists2Forall2Fold2.Tests.fs — 206 new tests: null checks, empty sequences (both/first/second empty), truncation at shorter sequence, type mixing, side-effect sequences, early-exit behaviour
• 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 exists2, fold2/fold2Async, forall2/forall2Async as ✅ in the table; update checklist

AI Disclosure

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

Test Status

✅ Build: succeeded (Release configuration, netstandard2.1)
✅ Tests: 4736 passed, 2 skipped (existing skips), 0 failed — full suite
✅ New tests: 206 tests across exists2, forall2/forall2Async, fold2/fold2Async
✅ Formatting: dotnet fantomas . --check passes

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