feat: add round history query

[TUPM96]

Closes #69

/claim #69

Summary

• add paginated get_round_history(group_id, offset, limit) for completed rounds
• return existing RoundInfo data including recipient, total contributed, deadline, and completion status
• skip incomplete active rounds and handle zero-limit/out-of-range pages
• add regression coverage for pagination and incomplete-round filtering

Validation

• cargo fmt --check
• cargo test --package sorosave (10 passed)
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Payout details can be provided privately on acceptance.

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