Add member reputation tracking

[big14way]

Track on-chain reputation for members based on contribution history across groups.

Acceptance Criteria:

• Add MemberReputation struct (groups_completed, on_time_contributions, defaults)
• Update on each contribution and payout
• Add get_reputation(member) query

Files: contracts/sorosave/src/types.rs, contracts/sorosave/src/storage.rs

[Scottcjn]

Implemented in PR #86: #86

This adds:

• MemberReputation storage keyed by member address
• on_time_contributions updates on each successful contribution
• groups_completed updates when a group finishes its full payout cycle
• get_reputation(member) with zeroed defaults for new members
• tests covering default state and lifecycle-driven updates

Note: defaults is included in the model and remains 0 for now because the current contract does not yet implement a missed-payment/default path.

Verified locally with cargo test.