[vault-contract] Add security-related tests

Based on `SECURITY_AUDIT_VAULT_CONTRACT-V1.0.0.md` (finding F-14).

**Severity:** Informational

## Summary
Add tests for overflow, constructor validation, TTL, and reentrancy scenarios.

## Acceptance criteria

- [ ] Test overflow in claim formula
- [ ] Test negative ROI in constructor (rejected)
- [ ] Test TTL behavior (when TTL is implemented)
- [ ] Tests pass