Implement partial contribution support

[big14way]

Allow members to make partial contributions when they cannot afford the full amount in one transaction.

Acceptance Criteria:

• Track cumulative contributions per member per round
• Allow multiple contribute calls until full amount is reached
• Mark contribution complete when total equals contribution_amount
• Add get_member_contribution_progress view function

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

[jiarongmai]

I can implement this feature. I have experience with similar tasks and can deliver high-quality code. Ready to start immediately!

[jiarongmai]

I can implement this feature. I have experience with similar tasks and can deliver high-quality code. Ready to start immediately!

[er1c-cartman]

Submitted PR: #145

Summary:

• Added 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 when cumulative progress reaches the required contribution amount.
• Added get_member_contribution_progress and regression coverage for partial completion.

Validation:

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

No secrets, tokens, or private runtime data are included.