runoff: reuse IRV Numba tally helpers for head-to-head round

[endolith]

Closes #3.

What changed

The contingent-vote / top-two runoff second round used a Python loop with list.index per ballot. That round is equivalent to “instant runoff among only the two finalists”: skip every other candidate on each ballot, then take first preference among {finalist_0, finalist_1}. That is exactly what _inc_rank_idx + _tally_at_rank_idx do—the same Numba path IRV uses.

Details

• Build eliminated_cands as all candidate IDs except the two finalists.
• Call _inc_rank_idx only when that set is non-empty (same Numba empty-set guard as IRV).
• Tally with _tally_at_rank_idx and read counts at the two finalist indices.

All 215 tests pass locally.

  

[what-the-diff]

PR Summary

• New Import Functions Introduced: The code now includes two new import functions called _inc_rank_idx and _tally_at_rank_idx from the _common module. These tools help streamline the process of updating ranks and tallying votes.

• Improved Vote Counting Process: The logic behind counting votes for two options, finalist_0 and finalist_1, has been refined. It now uses a method from the library NumPy, which simplifies calculations and makes the process more efficient than the former looping method.

• Introduction of eliminated_cands Set: A new set, eliminated_cands, was added. This serves as a check list to keep track of candidates who are no longer under consideration.

• Maximized use of New Methods: The newly added functions _inc_rank_idx and _tally_at_rank_idx are now being utilized for updating ranking index and tallying votes respectively. This change was made to make the overall workflow more efficient and reliable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.33%. Comparing base (8a7213f) to head (f66b32a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   96.31%   96.33%   +0.01%     
==========================================
  Files          19       19              
  Lines         489      491       +2     
==========================================
+ Hits          471      473       +2     
  Misses         18       18              

Flag	Coverage Δ
no-numba	95.51% <100.00%> (+0.01%)	⬆️
numba	89.20% <100.00%> (+0.04%)	⬆️

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