feat: track member reputation

[TUPM96]

/claim #21

Summary

• add MemberReputation with groups_completed, on_time_contributions, and defaults
• persist reputation by member address with zero-value defaults for new members
• increment on_time_contributions after each successful contribution
• increment groups_completed for every member when a group finishes its final payout
• expose get_reputation(member) for SDK/UI reads
• add tests for default reputation and lifecycle-driven reputation updates

Validation

• cargo fmt --check
• cargo test --package sorosave
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Payout details can be provided privately on acceptance.

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