Add delegated contribution support

[er1c-cartman]

Summary:

• Adds set_delegate, revoke_delegate, and contribute_for contract methods.
• Stores one active delegate per member per group.
• Lets the delegate pay while the contribution is credited to the group member.
• Adds regression coverage for delegate replacement, delegated contribution, round completion, and revocation.

Fixes #56.

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.