Add fuzz testing with property-based invariants

[Prasiejames]

Problem

Existing tests cover specific scenarios but do not explore the full state space. Critical invariants like "total supply always equals sum of all balances" or "dividend claims never exceed original amount" are not fuzzed.

Expected Behavior

A fuzz test harness that generates random sequences of operations and verifies invariants after each step.

Proposed Solution

Add a proptest or fuzz crate in the workspace that generates random command sequences:

enum FuzzOp {
    Mint(Address, i128),
    Transfer(Address, Address, i128),
    Approve(Address, Address, i128, u32),
    Burn(Address, i128),
    DistributeDividend(Address, i128),
    ClaimDividend(Address),
    SetPaused(bool),
    SetKycHook(Option<Address>),
    // ...
}

Invariants to verify after each step:

• total_supply == sum of all balances
• allowance <= max approved
• dividend_remaining + claimed == original_amount
• paused ? no transfers succeed : transfers may succeed
• holding_limit never exceeded

Affected Files

• New contracts/fuzz-tests/ or tests/fuzz/