Add partial contribution support

[er1c-cartman]

Summary:

• Adds contribute_partial(member, group_id, amount) while preserving existing full contribute() behavior.
• Tracks cumulative per-member contribution amounts per round.
• Marks a member complete only once their cumulative amount reaches the group contribution amount.
• Adds get_member_contribution_progress and regression coverage for partial completion.

Fixes #48.

Validation:

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

Notes:

• Generated build/test artifacts were not committed.
• No secrets, tokens, or private runtime data are included.