feat: add protocol fee configuration

[TUPM96]

/claim #18

Summary

• add protocol fee configuration backed by instance storage: protocol_fee_bps and protocol_treasury
• add protocol-admin setters/getters, including a 10,000 bps upper bound
• deduct the fee during distribute_payout, send it to the treasury, and pay the remainder to the round recipient
• add coverage for defaults, authorization, invalid fee rates, and payout fee accounting

Validation

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

Payout details can be provided privately on acceptance.

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