feat: add protocol fee configuration

[Scottcjn]

Summary

• add protocol fee configuration with a basis-point fee rate and treasury address
• initialize fee config in the constructor with 0 bps and the admin as default treasury
• add admin-only setters for protocol fee rate and treasury
• deduct the configured fee during payout distribution and send it to the treasury
• add tests for default config, admin-only updates, invalid fee rejection, and payout fee splitting

Notes

• Treasury is configurable so the fee destination can be tested and changed without coupling it to the protocol admin address

Testing

• cargo test

Closes #18

[a638011]

Closing: This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.

[a638011]

Withdrawing this PR. This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.