feat: track contribution defaults after deadlines

[TUPM96]

Closes #2
/claim #2

Summary

• add defaulted member tracking to each round
• reject late contributions after the round deadline
• add mark_defaults so missed contributors can be recorded after the deadline without accepting a late contribution
• emit a default event for each newly recorded missed contribution

Validation

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

Payout details can be provided privately on acceptance.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.