[Repo Assist] feat: add TaskSeq.replicate and TaskSeq.zip3 (60 tests)

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Summary

Adds two straightforward combinators that mirror their Seq/List/Array counterparts:

Function	Description
TaskSeq.replicate	Creates a task sequence by repeating a value n times
TaskSeq.zip3	Combines three task sequences into triples, truncating to the shortest

Both functions were listed in the README as "not yet implemented" (no note-1 restriction — they don't require pre-materialisation).

Implementation details

• replicate: validated count ≥ 0, implemented as a simple taskSeq { for _ in 1..count -> value }. Raises ArgumentException for negative counts (consistent with init).
• zip3: follows the same pattern as the existing zip — uses three async enumerators, advances all three in lockstep, stops when any is exhausted.

Test Status

✅ Build: dotnet build src/FSharp.Control.TaskSeq.sln -c Release — succeeded, 0 warnings
✅ Tests: dotnet test — 3910 passed, 2 skipped, 0 failed
✅ Format: dotnet fantomas . --check — passed

New tests (60 total)

• TaskSeq.Replicate.Tests.fs — 13 tests: empty (count=0), negative-count error, correct count/value, large count, null value, reuse of task seq, mutable value capture
• TaskSeq.Zip.Tests.fs — 47 tests added for zip3: null checks, empty sequences (all three positions), truncation to shortest, mixed types, side effects

Changes

• TaskSeqInternal.fs — replicate (after singleton), zip3 (after zip)
• TaskSeq.fsi — XML-doc signatures for both
• TaskSeq.fs — thin delegating members
• TaskSeq.Replicate.Tests.fs — new test file
• TaskSeq.Zip.Tests.fs — extended with zip3 tests
• FSharp.Control.TaskSeq.Test.fsproj — added new test file
• README.md — marked replicate and zip3 as ✅ implemented

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