Implement fee mechanism for protocol sustainability

[big14way]

Add an optional protocol fee (e.g., 0.5%) on each payout.

Acceptance Criteria:

• Add protocol_fee_bps to protocol config
• Deduct fee during distribute_payout
• Send fee to protocol treasury address
• Admin can update fee rate

Files: contracts/sorosave/src/payout.rs, contracts/sorosave/src/admin.rs

[Scottcjn]

Implemented in PR #87: #87

This adds:

• protocol fee config with protocol_fee_bps and treasury address
• constructor initialization with 0 bps and admin as default treasury
• admin-only setters for fee rate and treasury
• payout deduction that routes the configured fee to treasury before sending the remainder to the round recipient
• tests for default config, admin-only updates, invalid fee rejection, and payout fee splitting

Verified locally with cargo test.