fix(consensus): voter-list corruption and duplicate round-validator slots

[oXtxNt9U]

Summary

• _unvote: on head removal, clear the new head's prev pointer instead of the tail's. The old code corrupted the voter linked list, letting any voter crash nodes via a getVotes out-of-bounds panic.
• calculateRoundValidators: revert with InsufficientActiveValidators when there are fewer distinct active validators than the round size, instead of padding slots with duplicates — duplicate slots can never be signed (validatorIndex is not in the signed message), orphaning them and halting consensus.
• drop redundant persistent _shuffle() (-63% round-selection gas)

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.33%. Comparing base (6284884) to head (8bf6b65).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1349      +/-   ##
===========================================
+ Coverage    59.10%   59.33%   +0.22%     
===========================================
  Files          968      972       +4     
  Lines        65587    66016     +429     
  Branches      1835     1915      +80     
===========================================
+ Hits         38768    39168     +400     
- Misses       26579    26606      +27     
- Partials       240      242       +2     

Flag	Coverage Δ
contracts	93.00% <100.00%> (?)
packages	59.11% <ø> (+<0.01%)	⬆️

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