Add five-member lifecycle contract test

[klsk007]

Summary

• add an end-to-end contract test covering a five-member savings group
• run all five contribution and payout rounds
• verify each member receives exactly one payout, final balances return to the initial amount, and the group completes

Closes #32

Verification

• cargo fmt --all
• cargo test

[klsk007]

Hi! Just checking in on this PR. Could you please let me know whether this implementation matches the bounty requirements, or if any changes are needed before review/merge? Happy to adjust it quickly.