Implement locked balance invariant tests

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Implement invariant tests for locked balance behaviour.

Current Behaviour

Locked balances may not be consistently verified across deposit, lock, unlock, withdraw, and balance query flows.

Expected Behaviour

The contract should preserve balance invariants across valid and invalid operations.

Suggested Implementation

Create invariant tests for locked balance accounting and failure cases.

Files or Areas Likely Affected

• contracts/
• src/
• tests/
• docs/

Acceptance Criteria

• Locked balance invariants are documented.
• Deposit and lock interactions are tested.
• Withdraw and locked balance interactions are tested.
• Invalid operations do not corrupt state.
• Balance query results remain consistent.
• Docs reference invariant coverage.

Additional Notes

This is important for fund safety.