Feature/sc 005 verification aggregation engine

[Tobi-8]

Summary

• Add VerificationAggregator (contracts/VerificationAggregator.sol): a pure, integer-only consensus calculator that folds frozen effectiveStake votes from TruthBountyWeighted into trueWeight / falseWeight / totalWeight, resolves a canonical ClaimOutcome (VERIFIED_TRUE / VERIFIED_FALSE / INCONCLUSIVE), and stores an AggregationResult (with confidence in 0–10 000 bps) per claim.
• Deterministic by construction: the voter loop is order-independent (no sorting, no randomness); repeated calls return identical output.
• Tie handling returns INCONCLUSIVE (exact ties, zero participation); configurable minimum-participation thresholds (verification count, total weight, confidence) enforced with ThresholdNotMet.
• API: aggregateClaim(), getAggregation(), calculateWeights(), calculateConfidence(), isAggregated(); emits ClaimAggregated(claimId, outcome, confidence) for indexers.
• TruthBountyWeighted now implements IVerificationSource read getters (getClaimVoterCount, getClaimVoterAt, getVoteData); no storage layout changes, so future weighting models (reputation/quadratic/zk) can be layered without breaking compatibility.
• Engine is a pure calculator: it never transfers funds, slashes, or updates reputation.
• Documented gas benchmarks (docs/verification-aggregator-gas.md): aggregateClaim avg ~158k gas.
  Testing
• 25 new tests in test/VerificationAggregator.test.ts covering: unanimous/mixed/weighted-majority outcomes, confidence math, tie cases → INCONCLUSIVE, minimum-participation reverts, determinism (reversed vote order), repeat-aggregation guard, stress (10-verifier set, repeated claims), and access control.
• Full hardhat suite: 383 passing, 0 failing.

closes #285