feat(elections): IRV tabulation engine with test fixtures

Summary

Implement an Instant Runoff Voting tabulation engine that elects a single winner by iteratively eliminating the lowest-ranked candidate and redistributing those ballots.

Part of #1933.

Requirements

• Pure function: `tabulateIRV(ballots, candidates) → { rounds[], winner, exhaustedBallots }`
• Each round records: first-preference counts per candidate, eliminated candidate, ballots transferred
• Handle ties on elimination using a documented, deterministic tie-break (e.g., previous-round counts, then seeded random with election ID)
• Handle exhausted ballots (all ranked candidates eliminated) explicitly — count them, don't silently drop
• Majority threshold: winner must have > 50% of continuing ballots
• Edge cases covered: empty ballots, single-candidate race, all-same-ranking tie, every-ballot-exhausted scenario
• Property-based tests: monotonicity violations are known in IRV — document them, don't try to fix them
• Golden fixture tests against hand-calculated example elections
• Tabulation runs server-side only (Convex action), never in the client

Context

• Epic: #1933
• Depends on schema issue
• IRV handles single-winner races (Chair, President-elect, etc.)

Implementation notes

• Implement as a pure TS module in `packages/tabulation/` so it can be tested with vitest independent of Convex
• Compare output against a known reference implementation (e.g., `rcv` npm package) for at least 3 test cases

Start with the schema issue dependency and the implementation notes for packages/tabulation/. Define the pure TypeScript tabulation API and its tie-break and exhaustion behavior, then add Vitest property-based and golden fixture tests. Done means the edge cases are covered, results match the rcv reference for three cases, and tabulation runs only in the server-side Convex action.