[Dimank92]

Summary

Closes #199

Adds a differential property and fuzz testing harness for the vault contract:

• Randomized sequence generator over ordered operations: deposit, withdraw, transfer_tag, token allowlist toggles, integrator registration toggles, and cross-contract options settlement simulations.
• Dual-ledger differential testing: maintains an in-harness reference ledger, asserted for strict equality against on-chain contract state after every step.
• Tag Isolation Invariant: asserts that no operation on tag A modifies tag B's balance.
• Total Conservation Invariant: asserts that contract total escrowed equals the sum of individual tag balances across all tokens.
• Atomic Rollback Invariant: asserts that failing operations cause zero state changes (no partial balance mutations).
• Deterministic seed set: runs across seeds 1337, 42, 9999, 0xcafebabe, 0xdeadbeef for predictable, fast CI execution (<6s) plus a high-iteration #[ignore] test for nightly/extended fuzzing.
• Harness Soundness Test: includes negative test test_fuzz_harness_catches_tag_isolation_violation verifying that the harness reliably detects and fails closed on deliberate tag isolation tampering.

Test Verification

All 78 unit, adversarial, and fuzz tests in zenith-vault pass cleanly:

test result: ok. 78 passed; 0 failed; 11 ignored; 0 measured; 0 filtered out; finished in 5.88s

Payout Address

• Wallet: 0xC345C07229307d4f1c7935aD2cAe48e390215808
• Network: Polygon / Base / Arbitrum / Stellar