feat(elections): STV tabulation engine with test fixtures

Summary

Implement a Single Transferable Vote tabulation engine that elects multiple winners using the Droop quota and surplus transfer.

Part of #1933.

Requirements

• Pure function: `tabulateSTV(ballots, candidates, seats) → { rounds[], winners[], exhaustedBallots }`
• Compute Droop quota: `floor(validBallots / (seats + 1)) + 1`
• Elect any candidate reaching quota; transfer surplus using a documented method (Meek, Gregory, or Weighted Inclusive Gregory — pick and document one)
• When no candidate reaches quota, eliminate lowest and redistribute
• Handle tie-breaks deterministically (same rule as IRV)
• Handle exhausted ballots explicitly
• Stop when all seats filled, or when remaining candidates ≤ remaining seats (auto-elect)
• Document the chosen surplus-transfer method in code comments AND in `docs/elections/tabulation.md`
• Golden fixture tests with hand-calculated multi-seat elections
• Tabulation runs server-side only

Context

• Epic: #1933
• Depends on schema issue
• STV handles multi-seat races (board elections)
• IRV issue: see sibling issue

Implementation notes

• Start with Weighted Inclusive Gregory (WIG) — simpler to explain publicly than Meek, widely used (e.g., Scottish local elections)
• Same `packages/tabulation/` package as IRV
• Compare against a reference (e.g., OpenSTV) for at least 3 test cases

Start in the packages/tabulation/ package and read the IRV sibling issue and the schema issue first. Define the server-side STV API, then add hand-calculated golden fixtures and compare at least three cases with OpenSTV. Done means deterministic quota, transfers, eliminations, exhaustion, auto-election, and documented WIG behavior in code and docs/elections/tabulation.md.