Extract stateless IRV tabulation helpers

[fsargent]

Summary

• extract preference counting, elimination, vote ranking, and result conversion into pure module functions
• retain the existing Irv method API through static aliases
• replace the mutable keep={} default with None
• add focused helper-level regression coverage

Stack

Depends on #60. Retarget this PR to main after its parent merges.

Validation

• uv run python -m pytest (45 passed)
• trunk check

Summary by Sourcery

Extract stateless IRV tabulation helpers into module-level functions while preserving the existing Irv method API via static aliases.

Enhancements:

• Refactor IRV preference counting, candidate elimination, vote aggregation, and result conversion into pure helper functions reused by the Irv class.
• Adjust the IRV least-candidate helper to use a None default instead of a mutable set for the keep parameter.
• Expose the new IRV tabulation helpers for reuse in other parts of the codebase.

Tests:

• Add regression tests that exercise the IRV tabulation helper functions directly to ensure stateless behavior and correct vote tabulation.

[sourcery-ai]

Reviewer's Guide

Extracts IRV tabulation logic into stateless module-level helpers while preserving the existing Irv API via static aliases and adds focused regression tests for the helpers and keep semantics.

File-Level Changes

Change	Details	Files
Refactor IRV tabulation logic from instance methods into stateless module-level helper functions and wire them back into the Irv class via static method aliases.	Introduce build_preference_schedule, eliminate_candidate, candidate_votes, least_candidate, rank_vector_to_preference, and finish_order_to_results as pure functions operating on ballots, preference schedules, and finish orders. Replace Irv.buildPreferenceSchedule, Irv.eliminateCandidate, Irv.candidateVotes, Irv.getLeast, Irv.rankVectorToPreference, and Irv.finishOrderToResults implementations with staticmethod aliases to the new helpers. Normalize least_candidate keep parameter semantics by replacing the mutable dict default with a None default and handling it as an empty tuple internally.	src/vse_sim/methods/irv.py
Add regression tests that exercise the new stateless IRV helper functions and ensure existing behavior and contracts are preserved.	Import CandidateWithCount and the new IRV helper functions into the regression test module. Add test_irv_tabulation_helpers_are_stateless to validate preference schedule building, candidate vote ranking (including alternates with zero votes), least-candidate selection with and without keep, and candidate elimination aggregation. Keep existing IRV regression tests intact to validate overall results and strategy contracts.	tests/test_regressions.py

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider normalizing the eliminate_candidate API so it either always accepts a candidate ID or always requires CandidateWithCount, rather than silently returning the input when given the wrong type, to make misuse easier to detect.
• In least_candidate, you might want to document or enforce that keep is a set-like collection for efficient membership checks, since callers could currently pass slower container types without realizing the cost.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider normalizing the `eliminate_candidate` API so it either always accepts a candidate ID or always requires `CandidateWithCount`, rather than silently returning the input when given the wrong type, to make misuse easier to detect.
- In `least_candidate`, you might want to document or enforce that `keep` is a set-like collection for efficient membership checks, since callers could currently pass slower container types without realizing the cost.

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