Add yield source integration path

[SlaVKsVolks]

Summary

Implements the contract-side yield source integration path for #52.

• Adds a YieldSource trait in contracts/sorosave/src/yield.rs.
• Adds fixed-rate yield-source implementation/storage as the first integration target.
• Adds per-group yield_rate_bps configuration through set_yield_rate.
• Deposits idle funds into the yield source when a round becomes fully contributed.
• Withdraws yield position before payout and adds earned yield to the payout amount.
• Gracefully falls back to the normal contribution pot when no yield position exists.
• Adds a test proving configured yield is added to the payout.

Verification

• cargo fmt --all
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build

Closes #52