[Repo Assist] feat: add TaskSeq.zip4 (53 tests)

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What changed

Implements TaskSeq.zip4, which combines four task sequences into a task sequence of 4-tuples. The four sequences need not have equal lengths — when one sequence is exhausted any remaining elements in the other sequences are ignored.

TaskSeq.zip4 : TaskSeq<'T1>-> TaskSeq<'T2>-> TaskSeq<'T3>-> TaskSeq<'T4>-> TaskSeq<'T1 * 'T2 * 'T3 * 'T4>
```
This follows the exact same pattern as the existing `zip` and `zip3` functions. It was listed in the README roadmap table without a ✅.
## Files changed- `TaskSeqInternal.fs` — `zip4` implementation (mirrors `zip3`)- `TaskSeq.fs` — public static member wrapper- `TaskSeq.fsi` — XML-documented signature- `TaskSeq.Zip.Tests.fs` — 53 new tests covering null checks,empty variants,immutable sequences,side effects,unequal lengths,mixed types,andsingleton sequences- `README.md` — marks `zip4` as implemented (✅)- `release-notes.txt` — updated
## Test StatusBuild and full test suite verified:
```
Build succeeded.0 Warning(s)0 Error(s)Passed!-Failed:0,Passed:4590,Skipped:2,Total:4592

Formatting verified with dotnet fantomas . --check — no issues.

Trade-offs / Notes

• zip4 is a natural extension of zip3 with no design ambiguity.
• No Seq.zip4 exists in F# Core, so this is a TaskSeq-specific addition (as already indicated in the README roadmap table).
• The implementation is pure streaming with no materialisation; it advances all four enumerators in lockstep.

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