Feature/Referral fees by bps set up

[bathord]

Summary

• Introduced a configurable referral taker fee in bps as an alternative to the existing multiplier-based fee. Solves charging fees on whitelisted pools, where the multiplier approach doesn't work because taker fees are zero.
• Added ReferralTakerFeeConfig dynamic field to Pool, keyed by referral ID, to store per-referral taker fee rates (max 10 bps).
• Implemented update_pool_referral_taker_fee_rate to allow referral owners to set their bps taker fee rate.
• Enforced mutual exclusivity: multiplier can only be set when bps taker fee rate is absent or zero; bps taker fee rate can only be set when multiplier is zero.
• Updated process_referral_fees to apply exactly one fee mode: bps (fraction of traded volume) or multiplier (fraction of taker fees paid).
• 100% test coverage.