fuzz: add pause-invariant property test (closes #96)

[Degentle12]

Summary

Closes #96 — Property test: pause invariant.

Changes

• Added test_pause_invariant_all_user_ops_rejected proptest — pauses the contract and verifies all 5 state-changing user operations (register, tip, withdraw, update_profile, unregister) panic with Paused (Audit Contract Logic for Gas & Storage Optimization #10).
• Asserts on-chain state is unchanged after all rejected operations.
• Verifies operations succeed again after the admin unpauses.
• Added panic_message helper to extract panic payload strings.

Files

• src/fuzz.rs: +114 lines

Testing

cargo test test_pause_invariant

10,000 proptest cases.