test: add proptest invariants for VotingManager stake conservation (#607)

[Jonahadukwu5]

Closes #607

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Summary

Implements issue #607. Adds a proptest harness that asserts the global stake conservation invariant after every sequence of valid votes:

sum(outcome_distribution.values()) == market.total_staked == VotingStats.total_staked

Changes

• New: contracts/predictify-hybrid/src/voting_invariants.rs — 5 proptest suites (1000 cases each, deterministic seed)
• Modified: src/lib.rs — register #[cfg(test)] mod voting_invariants
• Fixed: require_auth_coverage_tests.rs — 4 pre-existing build errors (missing max_deviation_bps arg)

Test suites

Test	Coverage
prop_sequential_votes_conserve_stake	Invariant checked after every step in sequences of 1–20 votes
prop_multi_outcome_distribution_conserves_stake	All 4 outcome labels covered
prop_duplicate_user_stake_not_silently_doubled	Documents upsert/accumulate split; confirms AlreadyVoted guard is load-bearing
prop_zero_stake_does_not_corrupt_total	Zero-stake edge case
prop_large_voter_set_conserves_stake	50–200 voters, sum verified

Test output

test voting_invariants::prop_duplicate_user_stake_not_silently_doubled ... ok
test voting_invariants::prop_multi_outcome_distribution_conserves_stake ... ok
test voting_invariants::prop_sequential_votes_conserve_stake ... ok
test voting_invariants::prop_zero_stake_does_not_corrupt_total ... ok
test voting_invariants::prop_large_voter_set_conserves_stake ... ok

test result: ok. 608 passed; 0 failed; 0 ignored

Acceptance criteria

• ✅ 1000+ cases per invariant
• ✅ Failure shrinks to minimal counterexample (verified: duplicate test shrinks to stake1=1_000_000, stake2=1_000_000)
• ✅ Deterministic seed via source_file in ProptestConfig
• ✅ No unwrap() added to production code

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