[Merged by Bors] - support up to 4 poets in merged ATX

[poszu]

Motivation

To support merging identities running on up to 3 fallback poets (on different phase shifts).

Description

Extended the maximum size of Niposts slice in ATX V2 the wire type.

Test Plan

Updated tests to process a merged ATX with 4 poets.

TODO

• [x] Explain motivation or link existing issue(s)
• [x] Test changes and document test plan
• [ ] Update documentation as needed
• [ ] Update changelog as needed

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.7%. Comparing base (138c61c) to head (07be3fc). Report is 4 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #6316 +/- ## ========================================= - Coverage 81.8% 81.7% -0.1% ========================================= Files 312 312 Lines 34616 34616 ========================================= - Hits 28316 28299 -17 - Misses 4468 4479 +11 - Partials 1832 1838 +6 ```

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[poszu]

Bors merge

[spacemesh-bors[bot]]

Pull request successfully merged into develop.

Build succeeded:

• systest-status
• ci-status